The main effect of an independent variable is the effect of the variable averaging over all levels of other variables in the experiment. The means from the hypothetical experiment described in the section on factorial designs are reproduced below.

Dose | Simple Task |
Complex Task |
---|---|---|

0 mg |
32 |
80 |

100 mg |
25 |
91 |

200 mg |
21 |
96 |

The main effect of type of task is assessed by computing the mean for the two levels of type of task averaging across all three levels of dosage. The mean for the simple task is: (32 + 25 + 21)/3 = 26 and the mean for the complex task is: (80 + 91 + 95)/3 = 86.67. The main effect of type of task, therefore, involves a comparison of the mean of the simple task (26) with the mean of the complex task (86.67).

Analysis of variance provides a significance test for the main effect of each variable in the design.