Re: st: CI for adjusted mean

[Rachele Capocchi <rachele.capocchi@arsanita.toscana.it>]

Hi Ken,

sorry but maybe I don't understand what you explain in your mail.

I tried tried:

. adjust wei, by(rep) se ci

and I obtained the adjusted means of the groups, but I don't know how I 
can obtain the overall mean.
Is there a  command to do this ?

Thanks a lot,
Rachele

Rachele Capocchi
Agenzia Regionale Sanità Toscana
"U.O. Centro Statistico Elaborazione Dati"
Sede Operativa - viale Milton,7
50134 Firenze
Tel. 055-4624354
Fax. 055-4624345 


khigbee@stata.com ha scritto:

> Maarten buis <maartenbuis@yahoo.co.uk> 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    khigbee@stata.com
> StataCorp     1-800-STATAPC
>
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