Thanks Ken,
I understand my mistake now.
-- Maarten
--- [email protected] wrote:
> Maarten buis <[email protected]> asks:
>
> > ... I expected that this code:
> >
> > sysuse auto, clear
> > recode rep78 1/2 = 3
> > anova mpg rep78 wei, continuous(wei)
> > adjust wei, se ci
> >
> > would give me a mean of mpg adjusted for wei (with standard error
> and
> > confidence interval), but instead it gives me an empty table. Does
> > anyone have any thoughts on this one?
>
> The description paragraph for -adjust- (manual or help file) says:
>
> ... If by() is not specified, adjust produces results as if
> by() defined one group. Variables used in the estimation
> command but not included in either the by() variable list or
> the adjust variable list are left at their current values,
> observation by observation. Here adjust displays the average
> estimated prediction (or the corresponding probability or
> exponentiated prediction), substituting the mean of these
> unspecified variables within each group defined by the
> variables in the by() option.
>
> Since -by()- was not specified, we are dealing with one group
> (the whole estimation sample) and since rep78 was not specified
> in the -adjust- command, the mean of rep78 over the whole
> estimation sample is used in the prediction (see the last part of
> the description "... substituting the mean ...").
>
> With your example the mean of rep78 over the estimation sample is
> 3.57971. After an -anova- where rep78 is in the model as a
> categorical variable, -predict- (and hence -adjust- which uses
> -predict-) returns a missing value when asked for the prediction
> of the model where the categorical variable is set to a value
> that was not one of the levels of that categorical variable.
> That is why you end up with an empty table--the -predict- command
> returned a missing value for the prediction.
>
> You could instead do things like
>
> . adjust wei rep=3, se ci
> . adjust wei rep=4, se ci
> . adjust wei rep=5, se ci
>
> or more easily
>
> . adjust wei, by(rep) se ci
>
> With these cases, the rep78 variable is treated as a categorical
> variable (like it should be in this -anova- example).
>
> Ken Higbee [email protected]
> StataCorp 1-800-STATAPC
>
> *
> * For searches and help try:
> * http://www.stata.com/support/faqs/res/findit.html
> * http://www.stata.com/support/statalist/faq
> * http://www.ats.ucla.edu/stat/stata/
>
-----------------------------------------
Maarten L. Buis
Department of Social Research Methodology
Vrije Universiteit Amsterdam
Boelelaan 1081
1081 HV Amsterdam
The Netherlands
visiting address:
Buitenveldertselaan 3 (Metropolitan), room Z434
+31 20 5986715
http://home.fsw.vu.nl/m.buis/
-----------------------------------------
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*
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