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n-way Effect Plot

Because of aliasing, the -way effect plot is seldom used for fractional factorial designs, and when it is used it should be interpreted with care. Three- and four-factor interactions tend to be aliased with main effects or two-factor interactions, rendering the -way effect plot for these effects meaningless.

For example, the following plot suggests a significant effect (because the lines are crossed), but the three-factor interaction is aliased with SSPD, which the main-effects plot suggests is significant. Formal tests cannot unravel the two effects. If there is good reason to suspect that MTEMP*HTIME*GSIZE is active, separating it from SSPD will require more experimentation.