{smcl}
{* 15mar2002}{...}
{hline}
help for {hi:avplot3}
{hline}
{title:Partial regression plots for subsamples }
{p 8 16}{cmd:avplot3} {it:varname} {cmd:,}{cmdab:sep:arate(}{it:varname}{cmd:)} [ {it:graph_options} ]
{title:Description}
{p}{cmd:avplot3} generates "partial regression plots" from an analysis of covariance
model, where a category variable has been included in dummy-variable form among the regressors
along with a constant term (that is, a dummy for all but one categories appears) and the full set of
dummies have been interacted with a continuous variable. The "partial predictions" of
such a model are then defined, for each category, as the category-specific constant term
plus the category-specific slope multiplied by the continuous variable. Any number of additional
covariates, continuous or discrete, may appear in the regression model, but do not affect the
partial predictions.
{title:Remarks}
{p}One variable must be specified: the continuous variable which has been fully interacted
in the regression model. If there are G categories of the discrete variable, the continuous
variable must be included as G interaction terms, each named {it:categoryvarname} x {it:varname}.
See the example below.
{p}The separate option must specify the integer variable ({it:categoryvarname})
that defines the G categories. The model must contain (G-1) dummy variables for all but one
category, and a constant term. It is immaterial which category is excluded.
{title:Options}
{p 0 4}
{it:graph_options} are options of {cmd:graph, twoway}. One may use, for instance, the saving
option to create a .gph file from the displayed graph.
{p 4 4}
The defaults include {cmd:connect(l)} for each subset of predicted values.
{p 0 4}
{cmd:separate(}{it:varname}{cmd:)} specifies the integer variable which defines categories.
{title:Examples}
{p 4 8}{inp:. use auto}
{p 4 8}{inp:. drop if rep78==.}
{p 4 8}{inp:. recode rep78 1/2=2}
{p 4 8}{inp:. tab rep78,gen(repair)}
{p 4 8}{inp:. rename rep78 repair}
{p 4 8}{inp:. forv i=1/4 { g repair`i'xmpg = mpg*repair`i' } }
{p 4 8}{inp:. reg price length foreign repair2-repair4 repair1xmpg repair2xmpg repair3xmpg repair4xmpg }
{p 4 8}{inp:. avplot3 mpg, sep(repair) }
{title:Author}
Christopher F Baum, Boston College, USA
baum@bc.edu
{title:Acknowledgements}
Nick Cox and his routines regplot and keyplot were indispensable in constructing
this program. Development of the routine was inspired by conversations with Rebekah
Coley and Jodi Morris.
{title:Also see}
{p 0 19}On-line: help for {help regress}, {help graph}, {help regdiag},
{help regplot} (if installed), {help sparl} (if installed)
{p_end}
{p 1 19}Manual: {hi: [U] 21 Programming Stata}
{p_end}