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RE: st: How does Stata calculate percentiles?


From   Grace Jessie <gracejessie@hotmail.com>
To   <statalist@hsphsun2.harvard.edu>
Subject   RE: st: How does Stata calculate percentiles?
Date   Mon, 25 Oct 2010 13:41:51 +0000

Phil, Nick,
thank you for further help.
Nick, yes, there is loss of information. Thank you for reminder.

Grace

> Subject: Re: st: How does Stata calculate percentiles?
> From: philclayton@internode.on.net
> Date: Mon, 25 Oct 2010 17:12:15 +1100
> To: statalist@hsphsun2.harvard.edu
> 
> centile price, centile(`=100/3')
> 
> On 25/10/2010, at 4:05 PM, Grace Jessie wrote:
> 
> > Phil,Nick,
> > thank you a lot for replies.
> > -centile- works well.
> > However, does centile(33) equal to centile 100/3 I want to get?
> > Nick, thank you for reminder.I keep the variable and just do some statistics for the variable, so no loss of information.
> > Grace
> > 
> > ----------------------------------------
> >> From: n.j.cox@durham.ac.uk
> >> To: statalist@hsphsun2.harvard.edu
> >> Date: Sun, 24 Oct 2010 17:42:23 +0100
> >> Subject: RE: st: How does Stata calculate percentiles?
> >> 
> >> Phil gave a good answer. However, once you have the -centile- result in memory,
> >> 
> >> local cutpoint=r(c_1)
> >> recode price (min/`cutpoint'=0) (`cutpoint'/max=1), gen(pricecat)
> >> 
> >> can be replaced by one line
> >> 
> >> gen pricecat = (price >= r(c_1)) if !missing(price)
> >> 
> >> which yields 0, 1 and numeric missing as appropriate.
> >> 
> >> On the other hand, why do you want to throw away information like this?
> >> 
> >> Nick
> >> n.j.cox@durham.ac.uk
> >> 
> >> Phil Clayton
> >> 
> >> One way to do it would be to obtain the centile using the -centile- command, then -recode- the variable to create the indicator variable.
> >> 
> >> Example:
> >> sysuse auto
> >> centile price, centile(33)
> >> local cutpoint=r(c_1)
> >> recode price (min/`cutpoint'=0) (`cutpoint'/max=1), gen(pricecat)
> >> 
> >> If you wanted the indicator variable to be 1 if the variable is >= the cutpoint (as opposed to >), swap the two recoding rules (once one rule is matched, the subsequent rules are ignored).
> >> 
> >> See the manual for -centile- to see how it's calculated. It's pretty standard. With regards to "su varname, d", see -help summarize- and the manual for -summarize-
> >> 
> >> On 24/10/2010, at 3:27 PM, Grace Jessie wrote:
> >> 
> >>> I want to generate a new variable equaling 1 if the other variable is greater than its 100/3 percentile and 0 otherwise.How to get the 100/3th percentile of a variable?
> >>> And how does Stata calculate percentiles if the number of observations is odd or even?
> >>> Additionally, what does the output "smallest and largest" mean after "su varname,d"?
> >> 
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