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Re: st: How does Stata calculate percentiles?


From   Phil Clayton <philclayton@internode.on.net>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: How does Stata calculate percentiles?
Date   Mon, 25 Oct 2010 17:12:15 +1100

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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