What Is a Controlled Experiment?

What Is a Controlled Experiment?

A controlled experiment is one in which everything is held constant except for one variable. Usually, a set of data is taken for a control group, which is commonly the normal or usual state, and one or more other groups are examined, where all conditions are identical to the control group and each other except this one variable. Sometimes it's necessary to change more than one variable, but all of the experimental conditions will be controlled so that only the variables being examined change and the amount or way they change is measured.

Key Takeaways: Controlled Experiment

  • A controlled experiment is simply an experiment in which all factors are held constant except for one: the independent variable.
  • A common type of controlled experiment compares a control group against an experimental group. All variables are identical between the two groups except the factor being tested.
  • The advantage of a controlled experiment is that it makes it easier to eliminate uncertainty about significant of the results.

Example of a Controlled Experiment

Let's say you want to know if the type of soil affects how long it takes a seed to germinate. You decide to set up a controlled experiment to answer the question. You might take five identical pots, fill each with a different type of soil, plant bean seeds in each pot, place the pots in a sunny window, water them, and measure how long it takes for the seeds in each pot to sprout. This is a controlled experiment because your goal is to keep every variable constant except the type of soil you use. You control these things!

Why Controlled Experiments Are Important

The big advantage of a controlled experiment is you can eliminate much of the uncertainty about your results. If you couldn't control each variable, you might end up with a confusing outcome. For example, if you planted different types of seeds in each of the pots, trying to determine if soil type affected germination, you might find some types of seeds germinate faster than others. You wouldn't be able to say, with any degree of certainty, that the rate of germination was due to the type of soil! Or, if you had placed some pots in a sunny window and some in the shade or watered some pots more than others, you could get mixed results. The value of a controlled experiment is that it yields a high degree of confidence in the outcome.

Are All Experiments Controlled?

No, they are not. It's still possible to obtain useful data from uncontrolled experiments, but it's harder to draw conclusions based on the data. An example of an area where controlled experiments are difficult is human testing. Say you want to know if a new diet pill helps with weight loss. You can collect a sample of people, give each of them the pill, and measure their weight. You can try to control as many variables as possible, such as how much exercise they get or how many calories they eat. However, you will have several uncontrolled variables, which may include age, gender, genetic predisposition toward a high or low metabolism, how overweight they were before starting the test, whether they inadvertently eat something that interacts with the drug, etc. Scientists try to record as much data as possible when conducting uncontrolled experiments so they can see additional factors that may be affecting their results. Although it is harder to draw conclusions from uncontrolled experiments, new patterns often emerge that would not have been observable in a controlled experiment. For example, you may notice the diet drug seems to work for female subjects, but not for male subjects. This may lead to further experimentation and a possible breakthrough. If you had been able to perform a controlled experiment, perhaps only on male clones, you would have missed this connection.


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  • Robbins, H. (1952). "Some Aspects of the Sequential Design of Experiments". Bulletin of the American Mathematical Society. 58 (5): 527-535. doi:10.1090/S0002-9904-1952-09620-8