the factors of an experiment that are kept the same among all test groups
also called controls or constants
these factors are kept the same to ensure that the experiment results are due to changed in the independent variable.
Example: In an experiment scientists measure if fertilizer has an effect on plant growth. Some plants are given fertilizer and some are not. The height of the plants are recorded. The plants are given the same amount of water and are exposed to the same intensity of sunlight for the same time each day. The amount of water and sunlight are the controlled variables.