Learning Objectives Classify dependent, independent, and controlled variables. An experiment is a controlled method of testing a hypothesis. Experiments are designed to provide an opportunity to make observation that will help test a hypothesis. Experiments are best understood in term of three types of variables: independent variables, dependent variables, and controlled variables. It may be helpful to think of a variable simply as something that can be measured. Most experiments are repeated multiple times with slight variations. These repetitions are often called trials. A variable that is purposely altered between trials is called an independent variable . Usually, it is a best practice to have a single independent variable in an experiment. It may be helpful to think of the independent variable as the "input" of the experiment. If the independent variable is the "input" of an experiment, than the dependent variable is the "output." Dependent variables change in response to the independent variable (their name comes from the fact that they depend on the independent variable). Finally, some variables, called controlled variables , are kept constant through all of the trials. Controlled variables are kept constant so that their fluctuations do not alter the dependent variable and cloud its relationship with the independent variable. Sometimes a scatter plot (a mathematical diagram in which data is graphed against an x and a y axis) is used to display results from an experiment. If a scatter plot is used, the independent variable is generally plotted on the horizontal axis and the dependent variable is plotted on the vertical axis.