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Re: st: qplot , over()


From   Keith Dear <keithdear4@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: qplot , over()
Date   Sat, 24 Mar 2012 09:17:00 +1100

Thanks Nick, the new version does what I wanted.

In passing -- while I agree it's not hard to DIY this, in fact this is
a nice example of why it's better to use a provided (authorised)
program when possible.  Instead of your careful code, I would have
written:
. bys mpg (foreign): gen pp=(_n-0.5)/_N
It took me some thought to see why you didn't do it that way (they
give exactly the same result for the auto data).
Keith


On 23 March 2012 23:53, Nick Cox <njcoxstata@gmail.com> wrote:
> 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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-- 
Dr Keith Dear
National Centre for Epidemiology and Population Health
ANU College of Medicine, Biology and Environment
Australian National University
Canberra, ACT 0200 Australia
Phone +61 (02) 6273 2208

*
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