Thank you Ken.
So let me see if I understand, when I type -adjust
mpg,by( foreign)- , mpg is set to the overall mean for
mpg, and if I type -adjust,by(foreign)-, (without
specifying mpg) then mpg is set to the mean of mpg
within each foreign level. Correct?
Regarding my first question, I understand that it
depends on the question that I want answered, what I
don't understand is what question is being answered by
each of the two approaches. Specifically if I type:
. regress price mpg foreign
. adjust , by( foreign) se
or
. regress price mpg
. adjust, by( foreign) se
What are these two adjusts telling me? I am sorry if
this is to trivial, but I am confused about the
interpretation of the second approach.
Best,
Ricardo.
the adjusted
--- [email protected] wrote:
> Ricardo Ovaldia <[email protected]> asks:
>
> > I have two questions about -adjust- after
> -regress-.
> > First, under what circumstances should the -by()-
> > variable be included or not included in the
> regression
> > model. ( I get different adjust outputs),
>
> The answer depends on what question you are trying
> to ask of your
> data and regression model. The adjust output will
> vary depending
> on what you do, as it should.
>
> The way I would approach it is to first think of the
> appropriate
> regression model and run it. Then think about what
> kinds of
> summaries you wish to obtain from that model (i.e.,
> what question
> are you trying to answer). This is what guides how
> -adjust-
> should be used.
>
>
> > Second, according to the help file, the -se-
> option is
> > "equivalent to the stdp option of predict".
> Therefore,
> > I should be able to get what -adjust- reports
> using
> > -predict-, but I can't. Using the auto data and
> these
> > commands:
> >
> > qui regress price mpg foreign
> > predict yhat
> > predict se, stdp
> > adjust , by( foreign) se
> > by foreign, sort:sum yhat se
> >
> > I get that mean(yhat)= adjusted(mean) as expected,
> but
> > I can't get the adjusted(se). What am I missing?
>
>
> Try the following experiment to understand what
> Stata's -adjust-
> command is presenting in it's table.
>
> Using the auto dataset as the example,
>
> . sysuse auto
> . regress price mpg foreign
>
> Now add two observations to the bottom of the
> dataset
>
> . set obs 76
>
> And set foreign to 0 and 1 in the last two obs.
>
> . replace foreign = 0 in 75
> . replace foreign = 1 in 76
>
> And set mpg to the mean of mpg in the last two obs
> when foreign
> is 0 or 1 respectively.
>
> . summarize mpg if foreign==0 in 1/74
> . replace mpg = r(mean) in 75
> . summarize mpg if foreign==1 in 1/74
> . replace mpg = r(mean) in 76
>
> Now run your predicts
>
> . predict xb
> . predict se, stdp
>
> And look at what it produced in the last two obs.
>
> . list foreign mpg xb se in 75/76
>
>
> +------------------------------------------+
> | foreign mpg yhat se
> |
>
> |------------------------------------------|
> 75. | Domestic 19.8269 6072.423 350.979
> |
> 76. | Foreign 24.7727 6384.682 539.5994
> |
>
> +------------------------------------------+
>
> And then compare that with what -adjust- produced.
>
> . adjust in 1/74 , by(foreign) se
>
>
>
----------------------------------------------------------
> Dependent variable: price Command:
> regress
> Variable left as is: mpg
>
>
----------------------------------------------------------
>
> ----------------------------------
> Car type | xb stdp
> ----------+-----------------------
> Domestic | 6072.42 (350.979)
> Foreign | 6384.68 (539.599)
> ----------------------------------
> Key: xb = Linear Prediction
> stdp = Standard Error
>
>
> The reason why what you did was not equivalent for
> the stdp, was
> that the mean of the stdp is not necessarily the
> same as the stdp
> evaluated at the mean.
>
>
> Ken Higbee [email protected]
> StataCorp 1-800-STATAPC
>
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