Main effect

The main effect of an independent variable refers to the AVERAGE of the effect of the factor alone averaged across all levels of the factor. Remember that the main effects are investigated if and only if there is no interaction.

Example:
A cholesterol reduction clinic has one diet and one exercise regime. It was found that exercise alone was effective, and diet alone was effective in reducing cholesterol levels (main effect of exercise and main effect of diet). Also, for those patients who didn`t exercise, the diet worked equally well (main effect of diet); those who followed diet A and exercised got the benefits of both (main effect of diet A and main effect of exercise).