what is interaction plots useful for do interaction plots only apply to fixed effects? what is interaction plots used for which is useful to test whether interactions between random factors are fixed? why do we need an interaction plot why do interaction plots are useful for random factor analysis when should an interaction plot be used? when are interaction plots useful in modelling for a fixed effect which statement is true about an interaction plot? why is an interaction plot useful to test the validity of an experiment? are interaction plots useful for fixed or variable factors why use interpolation plots for interaction are interaction plots only useful when both effects are fixed? how are interaction plots useful why would a interaction plot have to be only used when two variables are fixed? are interaction plots useful why would you use a plot of interactions when is an interaction plot useful? why are interaction plots useful what is the purpose of interaction plots?