gd overview: routines for comparing groups with predicted probabilities
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The -gd- commands simplify the use of SPOST commands for group comparisons
in binary logit.

** gdaccum: accumulates predictions to a matrix. To initialize the matrix,
use -gdaccum- with the reset option. -gdprdif- accumlates predictions to
this matrix.

** gdset: indicates the name of the existing group variables, names to
group indicator variables to be created, and provides labels to name
the groups.

** gdgen: generates the group indicator variables named by -gdset- and
interactions between these variables and all RHS variables specified
by -gdgen-. After running -gdgen- the -logit- command is used to estimate
your model using the variables created by -gdgen-.

** gdxis: creates global variables with informatation that can be used
with -prvalue- or -gdprdif- to specify the values of the RHS variables
at which the predictions can be computed. These globals make it simple
to specific values of the RHS held at the overall means as well as at the
within group means.

** gdprdif: compute predicted probabilities at specified values of the
RHS variables and compute group differences. This command can use the globals
created byi -gdxis- and also use the options in -prvalue-.

Background
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The logit model for group g=1 is:

    Pr(Y=1|X,Z,g=1) = F(b1_g + b1_x X + b1_z Z)

and for group g=0:

    Pr(Y=1|X,Z,g=0) = F(b0_g + b0_x X + b0_z Z)

Traditional, Chow-type tests of the equality of coefficients across groups
such as:

    Ho: b1_x = b0_x

are inappropriate since they confound differences in the effects of the
variable X for the two groups with differences in unobserved heterogeneity
for the two groups (Allison 1999).

Test of the equality of predicted probabilities are not
affected by this identification problem (Long 20XX).

The -gd- commands use SPOST commands (Long and Freese 200x) to computed predicted
probabilities and test hypotheses of the form:

    Ho: Pr(Y=1|X=x,Z=z,G=1) = Pr(Y=1|X=x,Z=z,G=0)

EXAMPLE

See gd_eg1.do for example.

TO DO

Add: cross group means
    capture macro drop xisg0mn1 // RHS by group 0 interaction variables set to group 1 means.
    capture macro drop xisg1mn0 // RHS by group 1 interaction variables set to group 0 means.

HISTORY

2008-01-26
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