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Re: st: qplot , over()


From   Nick Cox <njcoxstata@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: qplot , over()
Date   Fri, 23 Mar 2012 12:53:25 +0000

By the way, a do-it-yourself approach is easier than might be thought

sysuse auto, clear
egen rank = rank(mpg), unique by(foreign)
egen n = count(mpg), by(foreign)
gen pp = (rank - 0.5 ) / n
separate mpg, by(foreign) veryshortlabel
scatter mpg? pp

SJ-7-2  gr0027  . .  Stata tip 47: Quantile-quantile plots without programming
        . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  N. J. Cox
        Q2/07   SJ 7(2):275--279                                 (no commands)
        tip on producing various quantile-quantile (Q-Q) plots

Note that this recipe is strong enough to cope with ties and missing
values and flexible in that you can insert your own definition of
plotting positions (-pp- above) if you don't like (i - 0.5) / n for
some reason.

Nick

On Fri, Mar 23, 2012 at 12:36 PM, Nick Cox <njcoxstata@gmail.com> wrote:
> In revising -qplot- for SJ 10-4 I inadvertently introduced a bug in
> implementation of the -over()- option. I thought no one had noticed
> but me, but Keith has been bitten by it.
>
> The bug has already been fixed in a update due shortly in SJ 12-1. I
> will send Keith a copy of the revised files.
>
> Nick
>
> On Fri, Mar 23, 2012 at 12:22 PM, Keith Dear <keithdear4@gmail.com> wrote:
>
>> How can I get several quantile plots in one diagram?
>> (There have been past threads on this, but I found no mention of what
>> I am getting.)
>>
>> sysuse auto
>> replace mpg=mpg/2 if foreign   // for greater separation
>> qplot mpg, by(foreign)      // two distinct curves, which I would like
>> to plot together
>> qplot mpg, over(foreign)  // points on a single curve, not what I expected
>>
>> Am I misunderstanding the help file? It seems to imply that by() and
>> over() give the same set of curves, which is what I want but isn't
>> what happens:
>>
>> ---- help qplot
>> by(varname[, suboptions]) specifies that calculations be carried out
>> separately for each distinct value of a
>>        specified single variable. Results will be shown separately in
>> distinct panels. See [G] by_option.
>> over(varname) specifies that calculations be carried out separately
>> for each distinct value of a specified
>>        single variable.  Curves will be shown together within the
>> same panel.  over() is only allowed with a single
>>        varname.
>> ----
>>
>> -qplot- is by NJC out of -net install gr42_5.pkg-

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