A controlled variable is the element or feature that cannot be changed during the course of an experiment. The controlled variable is kept constant so the changes in other variables can be observed more easily. According to About.com, it is also sometimes referred to as the "constant" variable
Scientific experiments typically involve a number of variables:
- Independent variable
- Dependent variable
- Controlled variable
An independent variable is one that is changed by the scientist in order to facilitate an action or reaction within the given experiment. An experiment is an example of a fair test when there is only one independent variable. In a fair test, only one element is changed by the scientist while the other elements remain the same.
The dependent variable is the one that changes as a result of the independent variable. The dependent variable is what gets measured during an experiment and what is affected by the other elements in the experiment.
The controlled variable is also called the constant variable. Whatever value has been assigned to the constant variable cannot be changed during the course of the experiment since doing so will create an unfair test. The controlled variable helps define the relationship between the independent and dependent variable in a given experiment. Depending on the nature of the experiment, there may be more than one controlled variable.
Science can be thought of as a method for reducing or eliminating the preconceived notions of the human being doing the experiments. Fallacious reasoning, such as confirmation bias, can sneak into even the most careful researchers' work, as described by How Stuff Works in relation to Gregor Mendel's seminal work in genetics. Sometimes, the desire to achieve a certain result damages the search for objectively true answers. By isolating just one variable--the height of the pea plant, for example--and controlling for other variables, such as total sunlight or rainfall, a scientist is able to eliminate much of the random noise that would otherwise obscure the phenomenon under investigation.
Drug trials are perhaps the most well-known arena for controlled experiments. As the FDA explains, volunteers to test new drugs are usually assigned to different groups before being issued the experimental medicine. One group is known as the control group and receives only a placebo. Any improvements reported by the control group cannot have been due to the drug, and so can be used to eliminate the "placebo effect" from the test group's results. In this case, the placebo is the control variable.