Stata's xi command performs dummy (indicator) coding on the fly and with the "*" operator allows for the interaction of two categorical variables or a categorical with a continuous variable. xi3 extends the capabilities of xi to include a number of additional coding systems and can create codings that allow for testing simple contrasts and simple effects. In addition to indicator coding, xi3 supports the following coding schemes: simple coding - compares each level to a reference level deviation coding - deviations from the grand mean Helmert coding - compares levels of a variable w/ the mean of subsequent levels reverse Helmert coding - compares levels of a variable w/ the mean of previous levels forward differences - adjacent levels, each vs. next backward differences - adjacent levels, each vs. previous orthogonal polynomial coding Additionally, xi3 supports user defined coding schemes which allow virtually any type of contrast to be used. Like xi, xi3 can be used in conjunction any of the estimation commands. xi3 will do three-way interactions with categorical variables, a mixture of categorical and continuous variables, or with continuous variables alone. xi3 can be issued as a stand alone command. In addition to the "*" operator for interactions, xi3 adds the "@" operator which performs the coding separately for each level of the second variable to allow for simple contrasts and simple effects.
