David Airey <[email protected]> asks about using the -by()- option,
with multiple overlays and the -if- option:
> .scatter lens_average eye_nolens if parity <= 9, by(parity)
>
> gives a 3 x 3 graph of nine plots (parity 1, 2 to 9)
>
> .tw lfit lens_a eye_n if parity <= 9, by(parity)
>
> gives a 3 x 3 graph of nine plot
>
> .tw (scatter lens_average eye_nolens if parity <= 9) ///
> (lfit lens_average eye_nolens if parity <= 9), by(parity)
>
> gives the message:
>
> . tw (scatter lens_average eye_nolens if parity <= 9) ///
> > (lfit lens_average eye_nolens if parity <= 9), by(parity)
> no observations
>
> but completes a graph (3 x 4) leaving three holes in positions 10-12.
>
> Is this behavior intended?
>
> Given the first two graphs, I would have thought the by() option would
> travel through the overlay of the plots properly. It doesn't, at least
> not the way I intended.
>
> Maybe I should have simply done,
>
> preserve
> drop if parity > 9
> tw (scatter lens_average eye_nolens) ///
> (lfit lens_average eye_nolens), by(parity)
> restore
So David would like to have each scatter plot in:
. tw scatter lens_average eye_nolens if parity <= 9, by(parity)
overlayed with the regression line(s) from (respectively):
. tw lfit lens_average eye_nolens if parity <= 9, by(parity)
And David tried:
. tw (scatter lens_average eye_nolens if parity <= 9) ///
(lfit lens_average eye_nolens if parity <= 9), by(parity)
but didn't get what he wanted.
In the first two graph commands the if-condition "if parity <= 9" applied to
the graph "globally", and thus restricted the data available to the -by()-
option. In the third command, the "if parity <= 9" conditions are now
"local", thus the -by()- option is not restricted to only work with the data
that allowed by the (albeit common) if conditions.
The solution for David's problem is to specify the -if- restriction globally:
. tw (scatter lens_average eye_nolens) ///
(lfit lens_average eye_nolens) ///
if parity <= 9, by(parity)
--Jeff
[email protected]
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