Scientists use controlled experiments to identify causative or correlative relationships between the independent and dependent variables. The scientist intentionally changes the independent variable to determine its effects on the dependent variable. The scientist measures the effects on the dependent variable. Other variables are held constant by the scientist to ensure that the response in the dependent variable reflects the changes in the independent variable.

For example, if a scientist wanted to determine the amount of fertilizer necessary to optimize plant growth, the scientist would change the amount of fertilizer provided to various plants. The scientist would then measure the growth of each plant. Additionally, the scientist would make sure that each plant received the same amount of sunlight and water. This ensures that only the amount of fertilizer is affecting plant growth. Thus, the amount of fertilizer is the independent variable, the plant growth is the dependent variable, and the amount of sunlight and water are other variables that are held constant during the controlled experiment.