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
simple coding - compares each level to a reference level
deviation coding - deviations from the grand mean
Helmert coding - compares levels of a variable with the mean of subsequent levels
reverse Helmert coding - compares levels of a variable with the mean of previous levels
forward differences - adjacent levels, each versus next
backward differences - adjacent levels, each versus 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 with any
of the estimation commands. xi3 will do three-way
interactions with categorical variables, a mixture of categorical and
continuous variables, or with continous 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.