Interaction contrast
= Interaction contrast
{title2=$(\mu_{1a}-\mu_{1b})-(\mu_{0a}-\mu_{0b})$}
An <interaction contrast> compares a treatment difference across levels of another factor. A nonzero difference of differences shows that an additive representation of those cell means is inadequate. Its <standard error> must use the error stratum in which that <interaction term> is randomized.