1.6: Scientific Experiments - Biology

1.6: Scientific Experiments - Biology

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Seeing Spots

The spots on this child's tongue are an early sign of vitamin C deficiency, which is also called scurvy. This disorder, which may be fatal, is uncommon today because foods high in vitamin C are relatively available. They include tomatoes, peppers, and citrus fruits such as oranges, lemons, and limes. However, scurvy was a well-known problem on navy ships in the 1700s. It was said that scurvy caused more deaths in the British fleet than did French and Spanish arms. At that time, the cause of scurvy was unknown and vitamins had not yet been discovered. Anecdotal evidence suggested that eating citrus fruits might cure scurvy. However, no one knew for certain until 1747, when a Scottish naval physician named John Lind did an experiment to test the idea. Lind's experiment was one of the first clinical experiments in the history of medicine.

What Is an Experiment?

An experiment is a special type of scientific investigation that is performed under controlled conditions. But unlike some other types of investigations, an experiment involves manipulating some factors in a system in order to see how it affects the outcome. Ideally, experiments also involve controlling as many other factors as possible in order to isolate the cause of the experimental results.

An experiment generally tests how one particular variable is affected by some other specific variable. The affected variable is called the dependent variable, or outcome variable. The variable that affects the dependent variable is called the independent variable. It is also called the manipulated variable because this is the variable that is manipulated by the researcher. Any other variables (control variable) that might also affect the dependent variable are held constant, so the effects of the independent variable alone are measured.

Lind's Scurvy Experiment

Lind began his scurvy experiment onboard a British ship after it had been at sea for two months and sailors had started showing signs of scurvy. He chose a group of 12 sailors with scurvy and divided the group into 6 pairs. All 12 sailors received the same diet, but each pair also received a different daily supplement to the diet (Table (PageIndex{1})).

Table (PageIndex{1}): Lind's Scurvy Experiment
Pair of SubjectsDaily Supplement to the Diet Received by this Pair
11 quart of cider
25 drops of sulfuric acid
36 spoons of vinegar
41 cup of seawater
52 oranges and 1 lemon
6spicy paste and a drink of barley water

Lind's experiment ended after just five days when the fresh citrus fruits ran out for pair 5. However, the two sailors in this pair had already fully recovered or greatly improved. The sailors in pair 1 (receiving the quart of cider) also showed some improvement, but sailors in the other pairs showed none.

Can you identify the independent and dependent variables in Lind's experiment? The independent variable is the daily supplement received by the pairs. The dependent variable is the improvement/non improvement in scurvy symptoms. Lind's results supported the citrus fruit cure for scurvy, and it was soon adopted by the British navy with good results. However, the fact that scurvy is caused by a vitamin C deficiency was not discovered until almost 200 years later.


Lind's scurvy experiment included just 12 subjects. This is a very small sample by modern scientific standards. The sample in an experiment or other investigation consists of the individuals or events that are actually studied. It rarely includes the entire population because doing so would likely be impractical or even impossible.

There are two types of errors that may occur by studying a sample instead of the entire population: chance error and bias.

  • A chance error occurs if the sample is too small. The smaller the sample is, the greater the chance that it does not fairly represent the whole population. Chance error is mitigated by using a larger sample.
  • Bias occurs if the sample is not selected randomly with respect to a variable in the study. This problem is mitigated by taking care to choose a randomized sample.

A reliable experiment must be designed to minimize both of these potential sources of error. You can see how the sources of error were addressed in another landmark experiment: Jonas Salk's famous 1953 trial of his newly developed polio vaccine. Salk's massive experiment has been called the "greatest public health experiment in history."

Salk's Polio Vaccine Experiment

Imagine a nationwide epidemic of a contagious flu-like illness that attacks mainly children and often causes paralysis. That's exactly what happened in the U.S. during the first half of the 20th century. Starting in the early 1900s, there were repeated cycles of polio epidemics, and each seemed to be stronger than the one before. Many children ended up on life support in so-called "iron lungs" (see photo below) because their breathing muscles were paralyzed by the disease.

Polio is caused by a virus, and there is still no cure for this potentially devastating illness. Fortunately, it can now be prevented with vaccines. The first polio vaccine was discovered by Jonas Salk in 1952. After testing the vaccine on himself and his family members to assess its safety, Salk undertook a nationwide experiment to test the effectiveness of the vaccine using more than a million schoolchildren as subjects. It's hard to imagine a nationwide trial of an experimental vaccine using children as "guinea pigs." It would never happen today. However, in 1953, polio struck such fear in the hearts of parents that they accepted Salk's word that the vaccine was safe and gladly permitted their children to participate in the study.

Salk's experiment was very well designed. First, it included two very large, random samples of children — 600,000 in the treatment group, called the experimental group, and 600,000 in the untreated group, called the control group. Using very large and randomized samples reduced the potential for chance error and bias in the experiment. Children in the experimental group were injected with the experimental polio vaccine. Children in the control group were injected with a harmless saline (saltwater) solution. The saline injection was a placebo. A placebo is a "fake" treatment that actually has no effect on health. It is included in trials of vaccines and other medical treatments so subjects will not know in which group (control or experimental) they have been placed. The use of a placebo helps researchers control for the placebo effect. This is a psychologically-based reaction to a treatment that occurs just because the subject is treated, even if the treatment has no real effect.

Experiments in which a placebo is used are generally blind experiments because the subjects are "blind" to their experimental group. This helps prevent bias in the experiment. Often, even the researchers do not know which subjects are in each group. This type of experiment is called a double-blind experiment because both subjects and researchers are "blind" to which subjects are in each group. Salk's vaccine trial was a double-blind experiment, and double-blind experiments are now considered the gold standard of clinical trials of vaccines, therapeutic drugs, and other medical treatments.

Salk's polio vaccine proved to be highly successful. Analysis of data from his study revealed that the vaccine was 80 to 90 percent effective in preventing polio. Almost overnight, Salk was hailed as a national hero. He appeared on the cover of Time magazine and was invited to the White House. Within a few years, millions of children had received the polio vaccine. By 1961, the incidence of polio in the U.S. had been reduced by 96 percent.

Limits on Experimentation

Well-done experiments are generally the most rigorous and reliable scientific investigations. However, their hallmark feature of manipulating variables to test outcomes is not possible, practical, or ethical in all investigations. As a result, many ideas cannot be tested through experimentation. For example, experiments cannot be used to test ideas about what our ancestors ate millions of years ago or how long-term cigarette smoking contributes to lung cancer. In the case of our ancestors, it is impossible to study them directly. Researchers must rely instead on indirect evidence, such as detailed observations of their fossilized teeth. In the case of smoking, it is unethical to expose human subjects to harmful cigarette smoke. Instead, researchers may use large observational studies of people who are already smokers, with nonsmokers as controls, to look for correlations between smoking habits and lung cancer.

Feature: Human Biology in the News

Lind undertook his experiment to test the effects of citrus fruits on scurvy at a time when seamen were dying by the thousands from this nutritional disease as he explored the world. Today's explorers are astronauts in space, and their nutrition is also crucial to the success of their missions. However, maintaining good nutrition in astronauts in space can be challenging. One problem is that astronauts tend to eat less while in space. Not only are they very busy on their missions, but they may also get tired of the space food rations. The environment of space is another problem. Factors such as microgravity and higher radiation exposure can have major effects on human health and require nutritional adjustments to help counteract them. A novel way of studying astronaut nutrition and health is provided by identical twin astronauts Scott and Mark Kelly, (Figure (PageIndex{3})).

The Kellys are the first identical twin astronauts, but twin studies are nothing new. Scientists have used identical (homozygotic) twins as research subjects for many decades. Identical twins have the same genes, so any differences between them generally can be attributed to environmental influences rather than genetic causes. Mark Kelly spent almost a full year on the International Space Station (ISS) between 2015 and 2016, while his twin, Scott Kelly, stayed on the ground, serving as a control in the experiment. You may have noticed a lot of media coverage of Mark Kelly's return to Earth in March 2016, because his continuous sojourn in space was the longest of any American astronaut at that time. NASA is learning a great deal about the effects of long-term space travel on the human body by measuring and comparing nutritional indicators and other health data in the twins.


  1. How do experiments differ from other types of scientific investigations?
  2. Identify the independent and dependent variables in Salk's nationwide polio vaccine trial.
  3. Compare and contrast chance error and bias in sampling. How can each type of error be minimized?
  4. What is the placebo effect? Explain how Salk's experimental design controlled for it.
  5. Fill in the blanks. The _____________ variable is manipulated to see the effects on the ___________ variable.
  6. True or False. In studies of identical twins, the independent variable is their genetics.
  7. True or False. Experiments cannot be done on humans.
  8. True or False. Larger sample sizes are generally better than smaller ones in scientific experiments.
  9. Answer the following questions about Lind’s scurvy experiment.
    1. Why do you think it was important that the sailors’ diets were all kept the same, other than the daily supplement?
    2. Can you think of some factors other than diet that could have potentially been different between the sailors that might have affected the outcome of the experiment?
    3. Why do you think the sailors who drank cider had some improvement in their scurvy symptoms?
  10. Explain why double-blind experiments are considered to be more rigorous than regular blind experiments.
  11. Why are studies using identical twins so useful?
  12. Do you think it is necessary to include a placebo (such as an injection with saline in a drug testing experiment) in experiments that use animals? Why or why not?

Explore More

Watch this entertaining TED talk, in which biochemist Kary Mullis talks about the experiment as the basis of modern science.

Check out this video to learn more about conducting scientific experiments:

Object Lessons

Heating a balloon with a candle pops an empty balloon, just as one sin hurts our connection to God. But a balloon filled with water will not pop, even when put directly into a candle flame. The protection conferred from the &lsquoliving water&rsquo is similar to protection from salvation both priceless yet free, both available to everyone, and both necessary for life, physical or eternal.

Just as David was able to defeat Goliath, so even the smallest person can out perform the biggest person in a competition to shoot Diet Coke out of a 2-liter, showing that no matter how big or small we are, we can accomplish great things with the help from God realized through prayer.

In this exercise, several unbelievable tasks will be performed that will be viewed as miraculous. These include opening a banana and finding it already sliced, submerging a hand in a jar of water and pulling it out completely dry, and using a playing card to prevent water spilling out of a glass of water even when completely turned upside-down and the card removed. However, these tasks are but tricks of science. The miraculous events in the bible, though, are true miracles, that by definition exceed scientific explanation. The belief that these biblical events occurred as written in the bible is pivotal to building a connection to God our faith in God is only as strong as our faith in the bible as God's word.

We all need to let the light of Jesus shine. If we let it extinguish, we become more susceptible to worldly things coming in to fill the void. The same is true in science. A lit candle that sits is a pool of water is extinguished when a vase is placed over it. The loss of the flame (heat) creates a void that is filled by the rising water. But having this light is not enough to fulfill God&rsquos plan, for many are needed to provide sufficient power to change the world. The same is true for science. This is called synergism and can be demonstrated using both a rubber ball and a bag experiment.

The &ldquoParable of the Sower&rdquo, as written in Mark 4:3-8, describes the word of God given to his people. The &ldquoParable of the Tenants&rdquo, as written in Matthew 25:14-30, describes our responsibility to grow His word. Just as seed sown from a farmer may fall on a path, rocky places, thorns, or good soil, so must we insure a fertile ground for His word to grow. Moreover, this word must be nurtured to insure maximal growth. This is only possible by eliminating Satan, establishing strong roots, not letting desires for worldly things come in to choke the word of God, being active in prayer, and going to church. If we insure good soil and nurture the word, the bible says the seed will produce a crop thirty, sixty or even a hundred times what was sown. In this demonstration, we sow our &ldquoseeds&rdquo in fertile soil, add nurturing agents, and watch the expansion of the word grow 100 times instantly.

Just as Daniel was protected from the lions and Shadrach, Meshach and Abednego were protected from the fiery furnace, so can sand be put underwater and remain dry and a cloth burn without igniting. While not always God&rsquos will to protect our earthly bodies, our heavenly bodies are protected by God by the sacrifice of his son Jesus, if we seek it.

Awesome High-Flying Science

Launching, throwing, blasting off, and sending things flying high into the sky can be fun for all ages. This week, we've got you covered with simple high-flying Awesome Summer Science Experiments that let kids experience the thrill of launching and flying with activities that are easy to do at home.

Independence Day (July 4) in the U.S. happens this week. While fireworks and the science behind their colors (see our 4th of July "rainbow fire" science experiment) can be lots of fun to explore, this week's Awesome Summer Science Experiments theme takes to the skies with activities that will fit right in with celebrations but are also fun anytime!

The activities in Week 4 of our Awesome Summer Science Experiments series feature various kinds of rockets, homemade kites, and a fun way to give your paper airplanes added oomph.

The Art & Science of Leaf Rubbings by Edventures with Kids

Texture rubbing is a staple in the art classroom, and I like how Jacquie incorporated the science of trees with her kids. I know I will definitely be using this activity in my homeschool!

How is the scientific method used by biologists?

Quick recap: Biologists and other scientists use the scientific method to ask questions about the natural world. The scientific method begins with an observation, which leads the scientist to ask a question. She or he then comes up with a hypothesis, a testable explanation that addresses the question.

A hypothesis isn’t necessarily right. Instead, it’s a “best guess,” and the scientist must test it to see if it’s actually correct. Scientists test hypotheses by making predictions: if hypothesis X is right, then Y should be true. Then, they do experiments or make observations to see if the predictions are correct. If they are, the hypothesis is supported. If they aren’t, it may be time for a new hypothesis.

Hypotheses are tested using controlled experiments

What are the key ingredients of a controlled experiment? To illustrate, let’s consider a simple (even silly) example.

Suppose I decide to grow bean sprouts in my kitchen, near the window. I put bean seeds in a pot with soil, set them on the windowsill, and wait for them to sprout. However, after several weeks, I have no sprouts. Why not? Well…it turns out I forgot to water the seeds. So, I hypothesize that they didn’t sprout due to lack of water.

To test my hypothesis, I do a controlled experiment. In this experiment, I set up two identical pots. Both contain ten bean seeds planted in the same type of soil, and both are placed in the same window. In fact, there is only one thing that I do differently to the two pots:

  • One pot of seeds gets watered every afternoon.
  • The other pot of seeds doesn’t get any water at all.

After a week, nine out of ten seeds in the watered pot have sprouted, while none of the seeds in the dry pot have sprouted. It looks like the “seeds need water” hypothesis is probably correct!

Let’s see how this simple example illustrates the parts of a controlled experiment.

Control and experimental groups

There are two groups in the experiment, and they are identical except that one receives a treatment (water) while the other does not. The group that receives the treatment in an experiment (here, the watered pot) is called the experimental group, while the group that does not receive the treatment (here, the dry pot) is called the control group. The control group provides a baseline that lets us see if the treatment has an effect. Controls can be positive controls to demonstrate that the process or treatment actually works, or they can be negative controls, where no change should occur during the experiment.

Independent and dependent variables

The factor that is different between the control and experimental groups (in this case, the amount of water) is known as the independent variable. This variable is independent because it does not depend on what happens in the experiment. Instead, it is something that the experimenter applies or chooses him/herself. Experiments can have more than one independent variable.

In contrast, the dependent variable in an experiment is the response that’s measured to see if the treatment had an effect. In this case, the fraction of bean seeds that sprouted is the dependent variable. The dependent variable (fraction of seeds sprouting) depends on the independent variable (the amount of water), and not vice versa.

Experimental data (singular: datum) are observations made during the experiment. In this case, the data we collected were the number of bean sprouts in each pot after a week.


In the scientific method, an experiment is an empirical procedure that arbitrates competing models or hypotheses. [2] [3] Researchers also use experimentation to test existing theories or new hypotheses to support or disprove them. [3] [4]

An experiment usually tests a hypothesis, which is an expectation about how a particular process or phenomenon works. However, an experiment may also aim to answer a "what-if" question, without a specific expectation about what the experiment reveals, or to confirm prior results. If an experiment is carefully conducted, the results usually either support or disprove the hypothesis. According to some philosophies of science, an experiment can never "prove" a hypothesis, it can only add support. On the other hand, an experiment that provides a counterexample can disprove a theory or hypothesis, but a theory can always be salvaged by appropriate ad hoc modifications at the expense of simplicity.

An experiment must also control the possible confounding factors—any factors that would mar the accuracy or repeatability of the experiment or the ability to interpret the results. Confounding is commonly eliminated through scientific controls and/or, in randomized experiments, through random assignment.

In engineering and the physical sciences, experiments are a primary component of the scientific method. They are used to test theories and hypotheses about how physical processes work under particular conditions (e.g., whether a particular engineering process can produce a desired chemical compound). Typically, experiments in these fields focus on replication of identical procedures in hopes of producing identical results in each replication. Random assignment is uncommon.

In medicine and the social sciences, the prevalence of experimental research varies widely across disciplines. When used, however, experiments typically follow the form of the clinical trial, where experimental units (usually individual human beings) are randomly assigned to a treatment or control condition where one or more outcomes are assessed. [5] In contrast to norms in the physical sciences, the focus is typically on the average treatment effect (the difference in outcomes between the treatment and control groups) or another test statistic produced by the experiment. [6] A single study typically does not involve replications of the experiment, but separate studies may be aggregated through systematic review and meta-analysis.

There are various differences in experimental practice in each of the branches of science. For example, agricultural research frequently uses randomized experiments (e.g., to test the comparative effectiveness of different fertilizers), while experimental economics often involves experimental tests of theorized human behaviors without relying on random assignment of individuals to treatment and control conditions.

One of the first methodical approaches to experiments in the modern sense is visible in the works of the Arab mathematician and scholar Ibn al-Haytham. He conducted his experiments in the field of optics—going back to optical and mathematical problems in the works of Ptolemy—by controlling his experiments due to factors such as self-criticality, reliance on visible results of the experiments as well as a criticality in terms of earlier results. He was one of the first scholars to use an inductive-experimental method for achieving results. [7] In his Book of Optics he describes the fundamentally new approach to knowledge and research in an experimental sense:

"We should, that is, recommence the inquiry into its principles and premisses, beginning our investigation with an inspection of the things that exist and a survey of the conditions of visible objects. We should distinguish the properties of particulars, and gather by induction what pertains to the eye when vision takes place and what is found in the manner of sensation to be uniform, unchanging, manifest and not subject to doubt. After which we should ascend in our inquiry and reasonings, gradually and orderly, criticizing premisses and exercising caution in regard to conclusions—our aim in all that we make subject to inspection and review being to employ justice, not to follow prejudice, and to take care in all that we judge and criticize that we seek the truth and not to be swayed by opinion. We may in this way eventually come to the truth that gratifies the heart and gradually and carefully reach the end at which certainty appears while through criticism and caution we may seize the truth that dispels disagreement and resolves doubtful matters. For all that, we are not free from that human turbidity which is in the nature of man but we must do our best with what we possess of human power. From God we derive support in all things." [8]

According to his explanation, a strictly controlled test execution with a sensibility for the subjectivity and susceptibility of outcomes due to the nature of man is necessary. Furthermore, a critical view on the results and outcomes of earlier scholars is necessary:

"It is thus the duty of the man who studies the writings of scientists, if learning the truth is his goal, to make himself an enemy of all that he reads, and, applying his mind to the core and margins of its content, attack it from every side. He should also suspect himself as he performs his critical examination of it, so that he may avoid falling into either prejudice or leniency." [9]

Thus, a comparison of earlier results with the experimental results is necessary for an objective experiment—the visible results being more important. In the end, this may mean that an experimental researcher must find enough courage to discard traditional opinions or results, especially if these results are not experimental but results from a logical/ mental derivation. In this process of critical consideration, the man himself should not forget that he tends to subjective opinions—through "prejudices" and "leniency"—and thus has to be critical about his own way of building hypotheses. [ citation needed ]

Francis Bacon (1561–1626), an English philosopher and scientist active in the 17th century, became an influential supporter of experimental science in the English renaissance. He disagreed with the method of answering scientific questions by deduction—similar to Ibn al-Haytham—and described it as follows: "Having first determined the question according to his will, man then resorts to experience, and bending her to conformity with his placets, leads her about like a captive in a procession." [10] Bacon wanted a method that relied on repeatable observations, or experiments. Notably, he first ordered the scientific method as we understand it today.

There remains simple experience which, if taken as it comes, is called accident, if sought for, experiment. The true method of experience first lights the candle [hypothesis], and then by means of the candle shows the way [arranges and delimits the experiment] commencing as it does with experience duly ordered and digested, not bungling or erratic, and from it deducing axioms [theories], and from established axioms again new experiments. [11] : 101

In the centuries that followed, people who applied the scientific method in different areas made important advances and discoveries. For example, Galileo Galilei (1564–1642) accurately measured time and experimented to make accurate measurements and conclusions about the speed of a falling body. Antoine Lavoisier (1743–1794), a French chemist, used experiment to describe new areas, such as combustion and biochemistry and to develop the theory of conservation of mass (matter). [12] Louis Pasteur (1822–1895) used the scientific method to disprove the prevailing theory of spontaneous generation and to develop the germ theory of disease. [13] Because of the importance of controlling potentially confounding variables, the use of well-designed laboratory experiments is preferred when possible.

A considerable amount of progress on the design and analysis of experiments occurred in the early 20th century, with contributions from statisticians such as Ronald Fisher (1890–1962), Jerzy Neyman (1894–1981), Oscar Kempthorne (1919–2000), Gertrude Mary Cox (1900–1978), and William Gemmell Cochran (1909–1980), among others.

Experiments might be categorized according to a number of dimensions, depending upon professional norms and standards in different fields of study.

In some disciplines (e.g., psychology or political science), a 'true experiment' is a method of social research in which there are two kinds of variables. The independent variable is manipulated by the experimenter, and the dependent variable is measured. The signifying characteristic of a true experiment is that it randomly allocates the subjects to neutralize experimenter bias, and ensures, over a large number of iterations of the experiment, that it controls for all confounding factors. [14]

Depending on the discipline, experiments can be conducted to accomplish different but not mutually exclusive goals: [15] test theories, search for and document phenomena, develop theories, or advise policymakers. These goals also relate differently to validity concerns.

Controlled experiments Edit

A controlled experiment often compares the results obtained from experimental samples against control samples, which are practically identical to the experimental sample except for the one aspect whose effect is being tested (the independent variable). A good example would be a drug trial. The sample or group receiving the drug would be the experimental group (treatment group) and the one receiving the placebo or regular treatment would be the control one. In many laboratory experiments it is good practice to have several replicate samples for the test being performed and have both a positive control and a negative control. The results from replicate samples can often be averaged, or if one of the replicates is obviously inconsistent with the results from the other samples, it can be discarded as being the result of an experimental error (some step of the test procedure may have been mistakenly omitted for that sample). Most often, tests are done in duplicate or triplicate. A positive control is a procedure similar to the actual experimental test but is known from previous experience to give a positive result. A negative control is known to give a negative result. The positive control confirms that the basic conditions of the experiment were able to produce a positive result, even if none of the actual experimental samples produce a positive result. The negative control demonstrates the base-line result obtained when a test does not produce a measurable positive result. Most often the value of the negative control is treated as a "background" value to subtract from the test sample results. Sometimes the positive control takes the quadrant of a standard curve.

An example that is often used in teaching laboratories is a controlled protein assay. Students might be given a fluid sample containing an unknown (to the student) amount of protein. It is their job to correctly perform a controlled experiment in which they determine the concentration of protein in the fluid sample (usually called the "unknown sample"). The teaching lab would be equipped with a protein standard solution with a known protein concentration. Students could make several positive control samples containing various dilutions of the protein standard. Negative control samples would contain all of the reagents for the protein assay but no protein. In this example, all samples are performed in duplicate. The assay is a colorimetric assay in which a spectrophotometer can measure the amount of protein in samples by detecting a colored complex formed by the interaction of protein molecules and molecules of an added dye. In the illustration, the results for the diluted test samples can be compared to the results of the standard curve (the blue line in the illustration) to estimate the amount of protein in the unknown sample.

Controlled experiments can be performed when it is difficult to exactly control all the conditions in an experiment. In this case, the experiment begins by creating two or more sample groups that are probabilistically equivalent, which means that measurements of traits should be similar among the groups and that the groups should respond in the same manner if given the same treatment. This equivalency is determined by statistical methods that take into account the amount of variation between individuals and the number of individuals in each group. In fields such as microbiology and chemistry, where there is very little variation between individuals and the group size is easily in the millions, these statistical methods are often bypassed and simply splitting a solution into equal parts is assumed to produce identical sample groups.

Once equivalent groups have been formed, the experimenter tries to treat them identically except for the one variable that he or she wishes to isolate. Human experimentation requires special safeguards against outside variables such as the placebo effect. Such experiments are generally double blind, meaning that neither the volunteer nor the researcher knows which individuals are in the control group or the experimental group until after all of the data have been collected. This ensures that any effects on the volunteer are due to the treatment itself and are not a response to the knowledge that he is being treated.

In human experiments, researchers may give a subject (person) a stimulus that the subject responds to. The goal of the experiment is to measure the response to the stimulus by a test method.

In the design of experiments, two or more "treatments" are applied to estimate the difference between the mean responses for the treatments. For example, an experiment on baking bread could estimate the difference in the responses associated with quantitative variables, such as the ratio of water to flour, and with qualitative variables, such as strains of yeast. Experimentation is the step in the scientific method that helps people decide between two or more competing explanations—or hypotheses. These hypotheses suggest reasons to explain a phenomenon, or predict the results of an action. An example might be the hypothesis that "if I release this ball, it will fall to the floor": this suggestion can then be tested by carrying out the experiment of letting go of the ball, and observing the results. Formally, a hypothesis is compared against its opposite or null hypothesis ("if I release this ball, it will not fall to the floor"). The null hypothesis is that there is no explanation or predictive power of the phenomenon through the reasoning that is being investigated. Once hypotheses are defined, an experiment can be carried out and the results analysed to confirm, refute, or define the accuracy of the hypotheses.

Experiments can be also designed to estimate spillover effects onto nearby untreated units.

Natural experiments Edit

The term "experiment" usually implies a controlled experiment, but sometimes controlled experiments are prohibitively difficult or impossible. In this case researchers resort to natural experiments or quasi-experiments. [16] Natural experiments rely solely on observations of the variables of the system under study, rather than manipulation of just one or a few variables as occurs in controlled experiments. To the degree possible, they attempt to collect data for the system in such a way that contribution from all variables can be determined, and where the effects of variation in certain variables remain approximately constant so that the effects of other variables can be discerned. The degree to which this is possible depends on the observed correlation between explanatory variables in the observed data. When these variables are not well correlated, natural experiments can approach the power of controlled experiments. Usually, however, there is some correlation between these variables, which reduces the reliability of natural experiments relative to what could be concluded if a controlled experiment were performed. Also, because natural experiments usually take place in uncontrolled environments, variables from undetected sources are neither measured nor held constant, and these may produce illusory correlations in variables under study.

Much research in several science disciplines, including economics, human geography, archaeology, sociology, cultural anthropology, geology, paleontology, ecology, meteorology, and astronomy, relies on quasi-experiments. For example, in astronomy it is clearly impossible, when testing the hypothesis "Stars are collapsed clouds of hydrogen", to start out with a giant cloud of hydrogen, and then perform the experiment of waiting a few billion years for it to form a star. However, by observing various clouds of hydrogen in various states of collapse, and other implications of the hypothesis (for example, the presence of various spectral emissions from the light of stars), we can collect data we require to support the hypothesis. An early example of this type of experiment was the first verification in the 17th century that light does not travel from place to place instantaneously, but instead has a measurable speed. Observation of the appearance of the moons of Jupiter were slightly delayed when Jupiter was farther from Earth, as opposed to when Jupiter was closer to Earth and this phenomenon was used to demonstrate that the difference in the time of appearance of the moons was consistent with a measurable speed.

Field experiments Edit

Field experiments are so named to distinguish them from laboratory experiments, which enforce scientific control by testing a hypothesis in the artificial and highly controlled setting of a laboratory. Often used in the social sciences, and especially in economic analyses of education and health interventions, field experiments have the advantage that outcomes are observed in a natural setting rather than in a contrived laboratory environment. For this reason, field experiments are sometimes seen as having higher external validity than laboratory experiments. However, like natural experiments, field experiments suffer from the possibility of contamination: experimental conditions can be controlled with more precision and certainty in the lab. Yet some phenomena (e.g., voter turnout in an election) cannot be easily studied in a laboratory.

An observational study is used when it is impractical, unethical, cost-prohibitive (or otherwise inefficient) to fit a physical or social system into a laboratory setting, to completely control confounding factors, or to apply random assignment. It can also be used when confounding factors are either limited or known well enough to analyze the data in light of them (though this may be rare when social phenomena are under examination). For an observational science to be valid, the experimenter must know and account for confounding factors. In these situations, observational studies have value because they often suggest hypotheses that can be tested with randomized experiments or by collecting fresh data.

Fundamentally, however, observational studies are not experiments. By definition, observational studies lack the manipulation required for Baconian experiments. In addition, observational studies (e.g., in biological or social systems) often involve variables that are difficult to quantify or control. Observational studies are limited because they lack the statistical properties of randomized experiments. In a randomized experiment, the method of randomization specified in the experimental protocol guides the statistical analysis, which is usually specified also by the experimental protocol. [17] Without a statistical model that reflects an objective randomization, the statistical analysis relies on a subjective model. [17] Inferences from subjective models are unreliable in theory and practice. [18] In fact, there are several cases where carefully conducted observational studies consistently give wrong results, that is, where the results of the observational studies are inconsistent and also differ from the results of experiments. For example, epidemiological studies of colon cancer consistently show beneficial correlations with broccoli consumption, while experiments find no benefit. [19]

A particular problem with observational studies involving human subjects is the great difficulty attaining fair comparisons between treatments (or exposures), because such studies are prone to selection bias, and groups receiving different treatments (exposures) may differ greatly according to their covariates (age, height, weight, medications, exercise, nutritional status, ethnicity, family medical history, etc.). In contrast, randomization implies that for each covariate, the mean for each group is expected to be the same. For any randomized trial, some variation from the mean is expected, of course, but the randomization ensures that the experimental groups have mean values that are close, due to the central limit theorem and Markov's inequality. With inadequate randomization or low sample size, the systematic variation in covariates between the treatment groups (or exposure groups) makes it difficult to separate the effect of the treatment (exposure) from the effects of the other covariates, most of which have not been measured. The mathematical models used to analyze such data must consider each differing covariate (if measured), and results are not meaningful if a covariate is neither randomized nor included in the model.

To avoid conditions that render an experiment far less useful, physicians conducting medical trials—say for U.S. Food and Drug Administration approval—quantify and randomize the covariates that can be identified. Researchers attempt to reduce the biases of observational studies with matching methods such as propensity score matching, which require large populations of subjects and extensive information on covariates. However, propensity score matching is no longer recommended as a technique because it can increase, rather than decrease, bias. [20] Outcomes are also quantified when possible (bone density, the amount of some cell or substance in the blood, physical strength or endurance, etc.) and not based on a subject's or a professional observer's opinion. In this way, the design of an observational study can render the results more objective and therefore, more convincing.

By placing the distribution of the independent variable(s) under the control of the researcher, an experiment—particularly when it involves human subjects—introduces potential ethical considerations, such as balancing benefit and harm, fairly distributing interventions (e.g., treatments for a disease), and informed consent. For example, in psychology or health care, it is unethical to provide a substandard treatment to patients. Therefore, ethical review boards are supposed to stop clinical trials and other experiments unless a new treatment is believed to offer benefits as good as current best practice. [21] It is also generally unethical (and often illegal) to conduct randomized experiments on the effects of substandard or harmful treatments, such as the effects of ingesting arsenic on human health. To understand the effects of such exposures, scientists sometimes use observational studies to understand the effects of those factors.

Even when experimental research does not directly involve human subjects, it may still present ethical concerns. For example, the nuclear bomb experiments conducted by the Manhattan Project implied the use of nuclear reactions to harm human beings even though the experiments did not directly involve any human subjects.

With all the rain we have been experiencing in the Triad lately, we decided it would be the perfect opportunity to have a lesson on the water cycle!

For this experiment you will need the following:

  • Plastic ziplock bag
  • Sharpie (to draw clouds and waves)
  • ¼ cup of water
  • Blue food coloring
  • Painter’s tape

Begin your experiment by drawing clouds around the top and water around the bottom of your plastic bag. This will serve as a visual aid of the water cycle and how it works.

Next, fill your plastic bag with ¼ cup of water, and add about 4 drops of food coloring.

Seal your bag shut, and hang it in a window (we recommend using painter’s tape since it is easy to remove once your experiment is over.)

Now it’s time to let nature run its course! Check on your bag periodically and notice how much condensation your baggie collects over time.

In nature, the sun’s heat causes water to evaporate from streams, lakes, rivers, and oceans. As the water vapor rises, it condenses to form clouds when it reaches cooler air. When the clouds are full of water, or saturated, they release some of the water as rain. Then the cycle starts over again.

The same principle can be applied to your experiment. Over the next few days, you will see that the water has warmed in the sunlight and evaporated into vapor. As that vapor cooled it began changing back into liquid, just like a cloud. When enough water condensed, the air couldn’t hold it anymore and the water fell down in the form of precipitation.

Remember, it is important to note that an experiment uses a variable (something that changes) to answer a question. To turn this demonstration into an experiment, you have to change something! Check out these questions to get you started:

  • Does the location (North facing, South facing, partial shade, full sun, etc) of the window have any impact on the cycle?
  • Does the amount of food coloring used have any impact?
  • How does the outside temperature impact the experiment?

Give it a try and let us know how your experiment turned out on our Facebook, Instagram, or Twitter pages using the hashtag #gscscience!

Landmark Experiments in Molecular Biology

Landmark Experiments in Molecular Biology critically considers breakthrough experiments that have constituted major turning points in the birth and evolution of molecular biology. These experiments laid the foundations to molecular biology by uncovering the major players in the machinery of inheritance and biological information handling such as DNA, RNA, ribosomes, and proteins. Landmark Experiments in Molecular Biology combines an historical survey of the development of ideas, theories, and profiles of leading scientists with detailed scientific and technical analysis.

Landmark Experiments in Molecular Biology critically considers breakthrough experiments that have constituted major turning points in the birth and evolution of molecular biology. These experiments laid the foundations to molecular biology by uncovering the major players in the machinery of inheritance and biological information handling such as DNA, RNA, ribosomes, and proteins. Landmark Experiments in Molecular Biology combines an historical survey of the development of ideas, theories, and profiles of leading scientists with detailed scientific and technical analysis.

Activities to Explore Chemical Reactions

1. Elephant Toothpaste

There is no actual toothpaste involved, but the Elephant Toothpaste reaction creates a fun, high-impact foaming demonstration of the reaction created when hydrogen peroxide is mixed with yeast and dish soap. (Optional: students can use various colors of food dye to create their own unique displays.)

2. Chemical Reaction Lava Lamp

In the Make an Alka-Seltzer Powered Lava Lamp activity, students can enjoy a groovy, bubbling lava lamp effect when they combine Alka-Seltzer®, mineral or vegetable oil, and water.

3. Lemon Volcano

In the Make a Lemon Volcano activity, students make their own fizzing volcanoes when they mix baking soda and lemon (citric acid) and see what the release of carbon dioxide (CO2) gas has to do with the volcano effect.

4. Invisible Ink

Have turmeric? In the Secret Messages With Invisible Ink! activity, students explore two different kinds of chemical reactions to see which works best for writing and decoding secret messages. (Tip! Get a firsthand look at how things went when this family did the activity at home.)

5. Bath Bombs

In the Making Homemade Bath Bombs activity, students explore different recipes and ingredients to see which will produce the fizziest bath bomb &mdash and why. (A convenient Bath Bomb Kit is available for students doing science projects about this chemical reaction.)

6. Exploring Enzymes

In the Exploring Enzymes activity, students learn about enzymes in the body and find out how the catalase enzyme helps protect cells.

7. Fire Snake

In the Make a Fire Snake activity, the combination of lighter fluid, sand, baking soda, and sugar triggers a chemical reaction in which an impressive fire snake seems to magically grow as it burns.

8. Cabbage Chemistry

In the Color-changing Cabbage Chemistry activity, students use cabbage to make an indicator solution and then learn about acids and bases by testing various foods and liquids.

9. Foamy Fake Snow

In the Foaming Fake Snow activity, students make fake snow and explore chemical reactions and surfactants.

Experiments on Biotechnology | Biology

Are you researching experiments on biotechnology ? You are in the right place. The below mentioned article includes a collection of seven experiments on biotechnology: 1. Culture Media 2. Callus Tissue Culture 3. Tissue Culture Medium 4. Tissue Culture in Plants 5. Isolation of Single Cells 6. Cell Planting Technique 7. Isolation of Protoplasm.

  1. Experiment on Culture Media
  2. Experiment on Callus Tissue Culture
  3. Experiment on Tissue Culture Medium
  4. Experiment on Tissue Culture in Plants
  5. Experiment on Isolation of Single Cells
  6. Experiment on Cell Planting Technique
  7. Experiment on Isolation of Protoplasm

1. Experiment on Culture Media:

Aim of the Experiment:

Prepare a list of nutritional requirements for root culture of Convolvulus arvensis.

Mix the above-mentioned macronutrient salts:

(A) micronutrient salts (B) and organic components (C). Adjust the pH of this medium to 4.5 and autoclave it for 15 minutes at 15 pounds per square inch.

2. Experiment on Callus Tissue Culture:

Aim of the Experiment:

Prepare a list of nutritional requirements of callus tissue culture of roots of Convolvulus arvensis.

2,4-Dichlorophenoxyacetic acid (2, 4-D), nicotinic acid, pyridoxine HCl, adenine sulphate, myoinositol and 1-glutamine.

List of nutritional requirements:

All components mentioned above in Experiment No. 1 plus some additional organic components mentioned below:

3. Experiment on Tissue Culture Medium:

Aim of the Experiment:

To prepare a tissue culture medium.

Tissue culture medium:

The simple method of preparing tissue culture medium is to use commercially available media of some good companies (e.g., SIGMA Chemical Company, St. Louis, USA or Hi Media Laboratories Pvt. Ltd., 23 Vadhani Industrial Estate, Bombay-86) in the market.

These are dry powdered media containing the desired macronutrients, micronutrients, vitamins and amino acids. The powder is dissolved in distilled water. Agar, sugar and other constituents are added in it, and distilled water is again added to prepare the final volume. The medium is autoclaved after the adjustment of the desired pH .

Composition of murashige-skoog medium:

Murashige-Skoog’s (MS) medium is the most widely used medium in the tissue culture laboratories for culturing plant tissue and cell culture of both monocotyledons and dicotyledons.

The composition of the various constituents of this medium is under mentioned:

4. Experiment on Tissue Culture in Plants:

Aim of the Experiment:

To work out the generalized steps used in the methodology of tissue culture in a plant material.

Plant material (e.g., mature carrot plant), water, scalpel or razor, cork borer, sterile petri-dishes, callus initiation medium (e.g., Murashige-Skoog’s medium) with 2,4-D, shoot development medium, pot with soil.

1. Take a mature carrot plant (Fig. 65 A) with its tap roots intact, remove its leaves and wash its tap roots thoroughly (Fig. 65 B).

2. Cut the tap root into 3 or 4 pieces (Fig. 65 C) with a sharp scalpel or razor.

3. Insert the cork borer into a tap root piece (Fig. 65 D) and take out the desired regions of root.

4. Put such a removed tap root piece in a sterile petri- dish and cut it transversely into small pieces as shown in Fig. 65 E.

Fig. 65. Various steps showing protocol for somatic embryogenesis in Carrot

5. Take some callus initiation medium (e.g., Murashige-Skoog’s medium or MS medium) with 2,4-D in a sterile petri-dish, place some discs or cut pieces of tap root on it and incubate for 6-8 weeks. Callus formation starts within 4-6 weeks (Fig. 65 F).

6. Transfer the callus to another petri-dish containing shoot development medium. Young plants with roots and shoots (Fig. 65G) start to develop within 4-8 weeks.

7. These young plants are transferred to pots containing soil (Fig. 65 H) where they develop into mature plants (Fig. 65 A).

5. Experiment on Isolation of Single Cells:

Aim of the Experiment:

To demonstrate the isolation of single cells from intact plant organs.

Fresh leaves of plant, 95% ethyl alcohol, calcium hypochlorite (7% solution), sterile distilled water, blade, potter-Elvehjem glass homogenizer tube, Rossini culture medium, sterile metal Tyler filters, centrifuge, agar plates, and incubator.

1. Take the fresh leaves and immerse them in 95% ethyl alcohol.

2. Rinse these leaves for 15 minutes in calcium hypochlorite solution (7%) and then wash 2-3 times in sterile distilled water.

3. Cut these leaves into small pieces of about 1 sq. cm., and put 1.5 gm. of such pieces in a potter- Elvehjem glass homogenizer tube.

4. Add 10 ml of Rossini culture medium into this homogenizer tube and homogenize the leaves.

5. Filter the medium containing homogenized leaves through two layers of such sterile metal Tyler filters of which the mesh diameter of upper layer is 61 mm and of lower layer is 38 mm.

6. Centrifuge the filtrate and discard the supernatant.

7. The sediment consists of free mesophyll cells. Suspend this sediment in a volume of medium.

8. Inoculate the free mesophyll cells into an agar plate or into the liquid medium and incubate these plates or vials in dark or light at 26°C.

Observations and results:

Sediment in the centrifuge tube contains free mesophyll cells. On a suitable medium these cells can be cultured.

6. Experiment on Cell Planting Technique:

Aim of the Experiment:

To demonstrate the Cell Planting Technique or process of Single Cell Culture and callus formation.

Free mesophyll cells (as obtained in Exercise No 5), beaker, Murashige-Skoog (MS) liquid medium (as prepared in Experiment No. 3), MS solid medium containing 0.6% agar, fine gauze, petri-dishes, sealing agent, inverted microscope, glass-marking pencil, incubator.

1. Take MS liquid medium in a beaker and suspend in it the free cells as obtained in Experiment No. 5.

2. Pass this cell suspension through a fine gauze.

3. In a separate beaker, dissolve MS solid medium and allow it to cool down to 35°C.

4. In a separate beaker, mix this molten MS agar medium and cell suspension in equal proportions (50: 50) and shake it well. By doing so the cells, would be evenly distributed throughout the medium.

5. Take some sterile petri-dishes and pour about 10 ml of this medium containing cells in each of them. Seal these petri-dishes with a sealing agent and incubate them in dark at about 25°C for 3-4 weeks.

Observations and results:

Prior to incubation, observe the single cells in the petri-dishes under an inverted microscope and mark the location of these cells on the outside of petri-dishes by a glass-marking pencil. This will make you sure about the isolation of pure single cells. After 3-4 weeks calli or colonies will develop on the agar surface in each petri-dish.

7. Experiment on Isolation of Protoplasm:

Aim of the Experiment:

Isolation of protoplast from different tissues using commercially available enzymes.

Root tips of Allium sativum, alcohol, distilled water, sodium hypochlorite, autoclave, mannitol, driselase enzyme, Knop’s solution, incubator, small sterile tubes, centrifuge, slides, microscope, agar- based culture medium, ultraviolet microscope.

Method and Observations:

1. Dip some young root tips of Allium sativum in 80% alcohol for 30 seconds and rinse them thoroughly with some sterile distilled water.

2. Now dip the root tips in 1.5% sodium hypochlorite for about 10 minutes and again rinse them thoroughly with sterile distilled water.

3. Repeat the rinsing process with distilled water 2-3 times.

4. Now cut the tips into small pieces in freshly prepared and autoclaved 0.5 M mannitol.

5. Prepare 5% stock solution of enzyme driselase by adding 2 ml of stock driselase in 18 ml of 0.5 M mannitol.

6. Now put the cut tips in 0.5% driselase for about 30 minutes.

7. Transfer the tips into a solution of mannitol and Knop’s solution (1:1) and incubate them at 37°C for about 15 hours.

8. The incubated tips are now taken in small sterile tubes to release the protoplast. Centrifuge them in mannitol two times for about 15 minutes at 1500 rpm.

9. After centrifuge process, discard the supernatant. The settled residue contains protoplasts.

10.Put a drop of this residue on a clean slide and observe under microscope carefully to see that cell wall has been removed.

The protoplasts have now been isolated. These isolated protoplasts can now be transferred to the culture medium for regeneration, and this process is called protoplast culture.

Now suspend the residue containing the isolated protoplasts in isotonic solution of mannitol. This will provide appropriate concentration of protoplast. This is now transferred to a suitable agar-based culture medium. Wait for a few hours.

The isolated protoplasts now begin to develop new cell wall, which can be detected by ultraviolet microscopy. The cells soon start to divide and form small callus colony. From the so-formed small colonies of callus, new intact plants can be regenerated.

Watch the video: AP Biology Sec - Science Practices u0026 Methods (June 2022).


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