A well-designed science experiment has well-defined controls and variables. But students of science aren’t always 100 percent sure which are which. Even high-level graduate students get this wrong, according to the University of Colorado. In this regard, examples can be informative and help students better understand the roles these important scientific ideas play in an experiment. Once you have a good understanding of what variables and controls are in scientific experiments, you’ll be able to pick them out of a research report without having them explicitly defined for you.

Variables and Controls

Variables come in two forms: dependent and independent. Most experiments have the goal of determining the relationship between the dependent and independent variables. The dependent variable is the variable the scientist is most interested in. Specifically, the scientist is interested in how his changes to the independent variable affect the dependent variable. The control is a variable held constant, thereby not being “variable” at all. The role of a control is to ensure that varying factors the scientist is not interested in and that would normally lead to changes in the dependent variable will not have an influence on the dependent variable in the experiment.

The Experiment “Depends” on You

All science experiments must have a dependent variable. To find an experiment’s dependent variable, you only need ask what the hypothesis is about. For example, an experiment testing the hypothesis “fertilizer A will increase the growth rate of sunflowers” is primarily interested in the effect on different growth rates, making “growth rate” the dependent variable. An experiment testing the hypothesis “drug A given to subjects directly increases the reaction time of the subjects” is primarily interested in the effect on reaction time, making “reaction time” the dependent variable.

Independent but Under Control

The variable under a scientist’s control is ironically not the experiment’s control variable but, instead, the “independent" variable. A scientist directly alters the independent variable during the experiment. For example, an experiment in which different types of diets are given to mice to see how long they live has "diet" as its independent variable, as the scientist has the power to directly change a mouse’s diet but not its lifespan. Likewise, in an experiment testing the relationship between the temperature of a room and the speed of evaporation of water in a cup, the temperature of the room is the independent variable, as the scientist can directly control it, but not the dependent variable -- how quickly the water evaporates.

A Control Is Such a Nuisance

The control in an experiment is a variable held constant due to its possible ability to be a nuisance in the experiment. In other words, it is a possible independent variable that the scientist has no interest in investigating. The scientist keeps the control at a single value so as not to have a varying impact on the experiment. An example of a control in an experiment investigating the relationship between diet and weight loss would be the amount exercise a subject gets. Because a scientist knows that exercise can lead to weight loss but is only interested in diet, he must ensure that all the subjects get the same amount of exercise to ensure that any difference in weight came from different diets.