What is "correct" will always depend on your view
of the processes generating the data, which are
not apparent.
It may be that what you want can be done cleanly
by
egen age_sex = group(age sex), label.
and then using age_sex as the basis for one set of
predictors.
Nick
[email protected]
Raoul C Reulen
> thanks for that.
>
> But I am still a bit confused about what to do in the following
> situation:
>
> .xi: regress BP i.age*group i.sex*group
>
> I want to know the difference between groupA and groupB for
> males in the
> first age group, then for females in the first age group,
> then for males
> in the second age group etc.? How do I do this? Simply by:
>
> .xi: reg BP group if age==1 & sex==0 (males, first age group)
>
> .xi reg BP group if age==1 & sex==1 (females, first age group)
>
> .xi reg BP group if age=2 & sex==0 (males, second age group)
>
> Or is this incorrect since you don't use the whole model? If correct,
> there must be an easier way to do this.
Joseph Coveney
> Raoul C Reulen wrote:
>
> I am comparing the blood pressure between two groups by means of
> multivariable regression. This is the regression model:
>
> .xi: regress BP i.age*group i.sex*group
>
> [omitted]
>
> . . . how do I stratify the variable 'group' also for 'sex' AND age?
> I want to now the regression coefficient of variable group, by age and
> sex.
>
> --------------------------------------------------------------
> ----------
> --------
>
> If you're looking only for two two-way interactions, then -xi- will do
> it without any trouble (see below).
>
> See also Michael Mitchell's & Phil Ender's -xi3-, especially if you
> eventually will want the third two-way interaction and the three-way
> interaction (although -xi- will do that, too; it just takes a
> couple of
> passes). Type:
>
> findit xi3
>
> on the command line form within Stata for more information.
> clear
> set more off
> sysuse bplong
> rename when group
> xi: regress bp i.agegrp*i.group i.sex*i.group // or
> xi3: regress bp i.agegrp*i.group i.sex*i.group // for the saturated
> model
> xi3: regress bp i.agegrp*i.sex*i.group
> exit
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