There must be an independent variable, which changes throughout the course of an experiment; a dependent variable, which is observed and measured; and a controlled variable, also known as the "constant" variable, which must remain consistent and unchanging throughout the experiment.

What is the constant variable in an experiment?

Controlled (or constant) variables: Are extraneous variables that you manage to keep constant or controlled for during the course of the experiment, as they may have an effect on your dependent variables as well.

What are the 3 constant variables?

An experiment usually has three kinds of variables: independent, dependent, and controlled.