Variables are factors or quantities that may be change or controlled in a scientific experiment. The three types of variables in a science project or experiment are independent, controlled and dependent variables. The independent variable is the condition or factor that is changed in the experiment and can affect the dependent variable.
A dependent variable is a factor that is observed or measured in the experiment. For a plant growth experiment, the maturation time is an example of a dependent variable. In this same experiment, the fertilizer type or temperature can be examples of independent variables.
The third type of variable is a controlled variable. This variable is constant in an experiment. Experiments are repeated a number of times to interpret how a change in the independent variable affects each trial result.