A main effect is a statistical term associated with experimental designs and their analysis. In the analysis of variance statistical test, which often is used to analyze data gathered via an experimental design, a main effect is the statistically significant difference between levels of an independent variable (e.g. mode of data collection) on a dependent variable (e.g. respondents' mean amount of missing data), ignoring the influence of other factors. To better understand the statistical concept of a main effect, it is helpful to understand a few key terms and experimental conditions under which a main effect may be found.

When conducting research, it is not uncommon to use a factorial analysis of variance to determine how two or more categorical independent variables (called factors in analysis ...