What are response and predictor variables?

Variables of interest in an experiment (those that are measured or observed) are called response or dependent variables. Other variables in the experiment that affect the response and can be set or measured by the experimenter are called predictor, explanatory, or independent variables.

For example, you might want to determine the recommended baking time for a cake recipe or provide care instructions for a new hybrid plant.

Subject Possible predictor variables Possible response variables
Cake recipe Baking time, oven temperature Moisture of the cake, thickness of the cake
Plant growth Amount of light, pH of the soil, frequency of watering Size of the leaves, height of the plant

A continuous predictor variable is sometimes called a covariate and a categorical predictor variable is sometimes called a factor. In the cake experiment, a covariate could be various oven temperatures and a factor could be different ovens.

Usually, you create a plot of predictor variables on the x-axis and response variables on the y-axis.