A variable is any factor, trait, or condition that can differ. An experiment usually has three kinds of variables: independent, dependent, and controlled.

  • The independent variable is controlled by the scientist.
  • The dependent variable changes, depending on the independent variable. This is measured by the scientist to prove or disprove their hypothesis.
  • The controlled variables are conditions which must be kept controlled and constant throughout the experiment so that they don't interfere with the dependent variable. When the experiment is repeated, the controlled variables must be exactly the same.

Diagram of variables in a drought experiment. A leafy plant grows in a pot, with a measuring stick being used to measure its growth. Liquid is being poured into the pot. The liquid is labelled as the independent variable, the thing that you manipulate in an experiment, and should be limited to one variable per experiment. The height of the plant is labelled as the dependent variable, the variable that depends on changes to the independent variable. The pot is labelled as a controlled variable, something that should remain constant and unchanging in the experiment.

Figure 1: Example of experimental variables for a drought experiment.