Controlled Experiment

Reviewed by: BD Editors

Controlled Experiment Definition

A controlled experiment is a scientific test that is directly manipulated by a scientist, in order to test a single variable at a time. The variable being tested is the independent variable, and is adjusted to see the effects on the system being studied. The controlled variables are held constant to minimize or stabilize their effects on the subject. In biology, a controlled experiment often includes restricting the environment of the organism being studied. This is necessary to minimize the random effects of the environment and the many variables that exist in the wild.

In a controlled experiment, the study population is often divided into two groups. One group receives a change in a certain variable, while the other group receives a standard environment and conditions. This group is referred to as the control group, and allows for comparison with the other group, known as the experimental group. Many types of controls exist in various experiments, which are designed to ensure that the experiment worked, and to have a basis for comparison. In science, results are only accepted if it can be shown that they are statistically significant. Statisticians can use the difference between the control group and experimental group and the expected difference to determine if the experiment supports the hypothesis, or if the data was simply created by chance.

Examples of Controlled Experiment

Music Preference in Dogs

Do dogs have a taste in music? You might have considered this, and science has too. Believe it or not, researchers have actually tested dog’s reactions to various music genres. To set up a controlled experiment like this, scientists had to consider the many variables that affect each dog during testing. The environment the dog is in when listening to music, the volume of the music, the presence of humans, and even the temperature were all variables that the researches had to consider.

In this case, the genre of the music was the independent variable. In other words, to see if dog’s change their behavior in response to different kinds of music, a controlled experiment had to limit the interaction of the other variables on the dogs. Usually, an experiment like this is carried out in the same location, with the same lighting, furniture, and conditions every time. This ensures that the dogs are not changing their behavior in response to the room. To make sure the dogs don’t react to humans or simply the noise of the music, no one else can be in the room and the music must be played at the same volume for each genre. Scientist will develop protocols for their experiment, which will ensure that many other variables are controlled.

This experiment could also split the dogs into two groups, only testing music on one group. The control group would be used to set a baseline behavior, and see how dogs behaved without music. The other group could then be observed and the differences in the group’s behavior could be analyzed. By rating behaviors on a quantitative scale, statistics can be used to analyze the difference in behavior, and see if it was large enough to be considered significant. This basic experiment was carried out on a large number of dogs, analyzing their behavior with a variety of different music genres. It was found that dogs do show more relaxed and calm behaviors when a specific type of music plays. Come to find out, dogs enjoy reggae the most.

Scurvy in Sailors

In the early 1700s, the world was a rapidly expanding place. Ships were being built and sent all over the world, carrying thousands and thousands of sailors. These sailors were mostly fed the cheapest diets possible, not only because it decreased the costs of goods, but also because fresh food is very hard to keep at sea. Today, we understand that lack of essential vitamins and nutrients can lead to severe deficiencies that manifest as disease. One of these diseases is scurvy.

Scurvy is caused by a simple vitamin C deficiency, but the effects can be brutal. Although early symptoms just include general feeling of weakness, the continued lack of vitamin C will lead to a breakdown of the blood cells and vessels that carry the blood. This results in blood leaking from the vessels. Eventually, people bleed to death internally and die. Before controlled experiments were commonplace, a simple physician decided to tackle the problem of scurvy. James Lind, of the Royal Navy, came up with a simple controlled experiment to find the best cure for scurvy.

He separated sailors with scurvy into various groups. He subjected them to the same controlled condition and gave them the same diet, except one item. Each group was subjected to a different treatment or remedy, taken with their food. Some of these remedies included barley water, cider and a regiment of oranges and lemons. This created the first clinical trial, or test of the effectiveness of certain treatments in a controlled experiment. Lind found that the oranges and lemons helped the sailors recover fast, and within a few years the Royal Navy had developed protocols for growing small leafy greens that contained high amounts of vitamin C to feed their sailors.

  • Field Experiment – An experiment conducted in nature, outside the bounds of total control.
  • Independent Variable – The thing in an experiment being changed or manipulated by the experimenter to see effects on the subject.
  • Controlled Variable – A thing that is normalized or standardized across an experiment, to remove it from having an effect on the subject being studied.
  • Control Group – A group of subjects in an experiment that receive no independent variable, or a normalized amount, to provide comparison.


1. Why is it necessary for scientist to conduct controlled experiments?
A. They allow for more definite relationships of cause and effect to be established
B. Without control, anything could happen in the experiment
C. Safety first! Controls are just silly safety precautions

Answer to Question #1
A is correct. A controlled experiment limits the massively changing world to one variable at a time, in increments determined by the experimenter. This allows for direct relationships between the variable and the changes in the subject to be established. In a field experiment, this is hard to do because many variables are changing constantly in nature. The controls in a controlled experiment have nothing to do with safety, nor do they regulate what can happen in an experiment. They are meant to ensure that only the independent variable is being observed, and that the experiment is completed successfully.

2. A population of birds is being studied. Which of the following describes a controlled experiment on the birds?
A. The birds are subjected to a lighting strike, and the results observed.
B. The birds are split in two groups, one left in the wild and one kept in captivity. After time, the effects are measured and compared.
C. The birds are observed as they migrate from North to South America.

Answer to Question #2
B is correct. The only option of the above choices that represents a controlled experiment is B. The other two answers describe observations of nature. Nothing in these answers was controlled by the experimenter. In answer B, the birds are being evaluated for differences caused by the environment of captivity. While this may seem like a wide variable, variables are defined by the experimenter and must simply have a group to contrast to. In this case, the control group would be the wild birds, who are not subjected to the differing conditions of captivity.

3. A scientist is interested in the effects of a pesticide on the eggs of certain birds. The scientist takes some eggs and coats them with pesticide, and does not coat a second group of eggs. He places them in an incubator for several days, then measures several chemical and physical properties of the eggs. Is this a good controlled experiment?
A. Yes
B. No
C. Maybe, but there may be more to the picture

Answer to Question #3
C is correct. While there is nothing specifically wrong with this experiment, it cannot be considered conclusive evidence of any effects seen in the eggs. Since birds lay their eggs in trees, it is unlikely that the pesticides spread on ground crops can end up coating an egg in nature. A better experiment would feed insects laced with the pesticide to birds while they produce eggs. This would be more representative of the natural mechanisms taking place, but still does not guarantee an answer. While scientists use controlled experiments to better understand the mechanisms of science, it still requires a body of evidence to support a theory.

Cite This Article

MLAAPAChicago Editors. "Controlled Experiment." Biology Dictionary,, 12 Feb. 2017, Editors. (2017, February 12). Controlled Experiment. Retrieved from Editors. "Controlled Experiment." Biology Dictionary., February 12, 2017.

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