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why do scientists use control groups in experiments

In an experiment, data from an experimental group is compared with data from a control group. These two groups should be identical in every respect except one: The difference between a control group and an experimental group is that the independent variable is changed for the experimental group, but is held constant in the control group. An is the group that receives an experimental procedure or a test sample. This group is exposed to changes in the being tested. The values of the independent variable and the result on the dependent variable are recorded. An experiment may include multiple experimental groups at one time. A control group is a group separated from the rest of the experiment such that the being tested cannot influence the results. This isolates the independent variable s effects on the experiment and can help rule out alternate explanations of the experimental results. While all experiments have an experimental group, not all experiments require. Controls are extremely useful where the
are complex and difficult to isolate. Experiments that use control groups are called. The most common type of control group is one held at ordinary conditions so it doesn t experience a changing variable. For example, If you want to explore the affect of salt on plant growth, the control group would be a set of plants not exposed to salt, while the experimental group would receive the salt treatment. If you want to test whether duration of light exposure affects fish reproduction, the control group would be exposed to a normal number of hours of light, while the duration would change for the experimental group.


Experiments involving human subjects can be much more complex. If you re testing whether a drug is effective or not, for example, members of a control group may expect they will not unaffected. To prevent skewing the results, a placebo may be used. A placebo is a substance that doesn t contain an active therapeutic agent. If a control group takes a placebo, participants don t know whether they are being treated or not, so they have the same expectations as members of the experimental group. However, there is also the placebo effect to consider. Here, the recipient of the placebo experiences an effect or improvement because she believes there should be an effect. Another concern with a placebo is that it s not always easy to formulate one that truly free of active ingredients. For example, if a sugar pill is given as a placebo, there s a chance the sugar will affect the outcome of the experiment. Positive control groups are control groups in which the conditions guarantee a positive result. Positive control groups are effective to show the experiment is functioning as planned. Negative control groups are control groups in which conditions produce a negative outcome. Negative control groups help identify outside influences which may be present that were not unaccounted for, such as contaminants.


A control group in a scientific experiment is a group separated from the rest of the experiment where the independent variable being tested cannot influence the results. This isolates the on the experiment and can help rule out alternate explanations of the experimental results. can also be separated into two other types: positive or negative. Positive control groups are groups where the conditions of the experiment are set to guarantee a positive result. A positive control group can show the experiment is functioning properly as planned. Negative control groups are groups where the conditions of the experiment are set to cause a negative outcome. Control groups are not necessary to all scientific experiments. Controls are extremely useful where the experimental conditions are complex and difficult to isolate. Negative control groups are particularly common in, to teach students how to identify the independent variable. An simple example of a control group can be seen in an experiment in which the researcher tests whether or not a new fertilizer has an effect on plant growth. The negative control group would be the set of plants grown without the fertilizer, but under the exact same conditions as the experimental group. The only difference between the experimental group would be whether or not the fertilizer was used. There could be several experimental groups, differing in the concentration of fertilizer used, its method of application, etc.


The would be that the fertilizer has no effect on plant growth. Then, if a difference is seen in the growth rate of the plants or the height of plants over time, a strong correlation between the fertilizer and growth would be established. Note the fertilizer could have a negative impact on growth rather than a positive impact. Or, for some reason, the plants might not grow at all. The negative control group helps establish that the experimental variable is the cause of atypical growth, rather than some other (possibly unforeseen) variable. A positive control demonstrates an experiment is capable of producing a positive result. For example, let s say you are examining bacterial susceptibility to a drug. You might use a positive control to make sure the growth medium is capable of supporting any bacteria. You could culture bacteria known to carry the drug resistance marker, so they should be capable of surviving on a drug-treated medium. If these bacteria grow, you have a positive control that shows other drug-resistance bacteria should be capable of surviving the test. The experiment could also include a negative control. You could plate bacteria known not to carry a drug resistance marker. These bacteria should be unable to grow on the drug-laced medium. If they do grow, you know there is a problem with the experiment.

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