Main Effects Plot

A main effects plot is used to examine differences between level means for one or more factors. There is a main effect when different levels of a factor affect the response differently. A main effects plot graphs the response mean for each factor level connected by a line. In other words, it is a plot of means at the various levels of each factor compared to the overall mean.

Example

For example, fertilizer company B is comparing the plant growth rate measured in plants treated with their product compared to plants treated by company A’s fertilizer. They tested the two fertilizers in two locations. The following are the main effects plots of these two factors.

Main Effects Plot example
Fertilizer seems to affect the plant growth rate because the line is not horizontal. Fertilizer B has a higher plant growth rate mean than fertilizer A. Location also affects the plant growth rate. Location 1 had a higher plant growth rate mean than location 2. The reference line represents the overall mean.

General patterns to look for:

  • When the line is horizontal (parallel to the x-axis), then there is no main effect. Each level of the factor affects the response in the same way, and the response mean is the same across all factor levels.
  • When the line is not horizontal, then there is a main effect. Different levels of the factor affect the response differently. The steeper the slope of the line, the greater the magnitude of the main effect.

Main effects plots will not show interactions. To view interactions between factors, use an interaction plot.

References

Minitab Support. What is a main effects plot? – Minitab