Controlled Experiment - Definition and Examples | Dictionary of Biology (2023)

Controlled trial definition

A controlled experiment is a scientific test directly manipulated by a scientist to test a single variable at a time. The variable tested is theindependent variable, and is adjusted to see the impact on the system under study. Thecontrolled variablesheld constant to minimize or stabilize their effects on the subject. In biology, a controlled experiment often involves constraining 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 specific variable, while the other group receives a default environment and conditions. This group is referred to ascontrol group, and allows for a comparison with the other group known as theexperimental group. There are many types of controls in different experiments to ensure that the experiment worked and to have a basis for comparison. In science, results are only accepted if they can be provenstatistically significant. Statisticians can use the difference between the control group and experimental group and the expected difference to determine whether the experiment supports thishypothesis, or if the data simply arose by accident.

Examples of controlled experiments

Music preference in dogs

Do dogs have a taste in music? You may have thought about it, and so has science. Believe it or not, researchers have actually tested dogs' responses to different genres of music. To set up a controlled experiment like this, the scientists had to consider the many variables that affect each dog during the test. The environment the dog is in when listening to music, the volume of the music, the presence of people, and even temperature were all variables the researchers had to consider.

In this case, the genre of the music was the independent variable. In other words, to see whether dogs change their behavior in response to different types of music, a controlled experiment needed to constrain the interaction of the other variables to the dogs. Such an experiment is usually always carried out in the same place, with the same lighting, the same furniture and the same conditions. This ensures that the dogs don't change their behavior in response to the space. To ensure the dogs don't react to people or simply the sound of the music, no one is allowed in the room and the music must be played at the same volume for each genre. Scientists will develop protocols for their experiment that ensure many other variables are controlled.

This experiment could also split the dogs into two groups and only test the music of one group. The control group would be used to set a baseline behavior and see how dogs behave without music. The other group could then be observed and the differences in the behavior of the group analyzed. By rating behavior on a quantitative scale, statistics can be used to analyze the difference in behavior and determine whether it was large enough to be considered significant. This fundamental experiment was conducted on a large number of dogs and their behavior with a variety of different music genres was analyzed. Dogs have been found to exhibit more relaxed and calm behavior when a certain type of music is played. Find out, dogs like reggae the most.

Scurvy in Sailors

In the early 1700s the world was a rapidly expanding place. Ships were built and sent all over the world carrying thousands upon thousands of sailors. These sailors were mostly fed the cheapest fare, not only because it reduced the cost of goods, but also because it is very difficult to keep fresh food at sea. Today we understand that deficiencies in 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 are just a general feeling of weakness, the prolonged lack of vitamin C causes the blood cells and vessels that carry blood to break down. As a result, blood leaks out of the vessels. Eventually people bleed out internally and die. Before controlled experiments became commonplace, a humble doctor decided to tackle the problem of scurvy. James Lind of the Royal Navy devised a simple controlled experiment to find the best cure for scurvy.

He divided sailors with scurvy into different groups. He subjected them to the same controlled conditions and gave them the same diet, except for one item. Each group underwent a different treatment or remedy taken with their food. Some of these remedies were barley water, cider, and a regiment of oranges and lemons. This created the firstclinical study, or testing the effectiveness of certain treatments in a controlled experiment. Lind found that the oranges and lemons helped sailors recover quickly, and within a few years the Royal Navy had developed protocols for growing small leafy greens containing large amounts of vitamin C to feed their sailors.

  • field trial– An experiment in nature, beyond the limits of total control.
  • Independent variable– The thing in an experiment that is 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 prevent it from affecting the subject being studied.
  • control group– A group of subjects in an experiment who receive no independent variable or a normalized amount for comparison.


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

Answer to question #1

Ait's right. A controlled experiment confines the massively changing world to one variable at a time, in increments determined by the experimenter. This allows direct relationships to be established between the variable and changes in the subject. in onefield trial, this is difficult because many variables in nature are constantly changing. The controls in a controlled experiment have nothing to do with safety, nor do they govern what can happen in an experiment. Their purpose is to ensure that only the independent variable is observed and that the experiment completes successfully.

2. A bird population is studied. Which of the following statements describes a controlled experiment on the birds?
A.The birds are exposed to a lightning strike and the results are observed.
B.The birds are divided into two groups, one in the wild and one in captivity. After some time, the effects are measured and compared.
C.The birds are observed on their migration from North to South America.

Answer to question #2

Bit's right. The only option of the choices above 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 examined for differences caused by the environment of captivity. While this may seem like a broad variable, variables are defined by the experimenter and simply need to have a group to contrast with. In this case, the control group would be the wild birds that are not exposed to the different husbandry conditions.

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 batch of eggs. He puts them in an incubator for several days and then measures various chemical and physical properties of the eggs. Is this a good controlled experiment?
C.Maybe, but maybe there is more to the picture

Answer to question #3

Cit's right. Although there is nothing particularly wrong with this experiment, it cannot be taken as conclusive evidence of any effects observed in the eggs. Since birds lay their eggs on trees, the pesticides applied to soil crops are unlikely to coat an egg in nature. A better experiment would feed birds insects spiked with the pesticide 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 mechanics of science, it still takes a lot of evidence to support a theory.

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