# Re: st: test difference in quintiles

 From Susanne Neckermann To statalist@hsphsun2.harvard.edu Subject Re: st: test difference in quintiles Date Wed, 10 Jun 2009 17:37:47 +0200

```Dear T

thank you very much for the message!!
```
I am a real beginner with Stata and I have to admit that I do not quite understand how to transfer the code to my case. I have a variable X that I observe under two conditions Y and Z. I want to see if the fifth percentile of variable X in condition Y is different from the fifth percentile of X in condition Z.
```
Thank you so much for your input!!
Best
Susanne

On 10-Jun-09, at 5:26 PM, Tirthankar Chakravarty wrote:

```
```<>

This is a variation of previous advice:
http://www.stata.com/statalist/archive/2009-05/msg00311.html

I believe you want to test the significance of the difference of the
5th percentile for two variables using a bootstrap. If this is the
case, then the code below applies:

/* Bootstrap the difference of percentiles */
clear
webuse nlswork, clear
keep idcode year ttl_exp
reshape wide ttl_exp, i(idcode) j(year)

cap prog drop pctile_bs
program define pctile_bs, rclass
version 10
syntax varlist(max=2 min=2) [if] [in]
marksample touse
tokenize `varlist'
local first `1'
macro shift
local second `*'
_pctile `first' if `touse', percentiles(5)
scalar pc1 = r(r1)
_pctile `second' if `touse', percentiles(5)
scalar pc2 = r(r1)
return scalar dpc = pc1-pc2
ereturn post, esample(`touse')
end
```
bootstrap Dpc = r(dpc), reps(2000) dots: pctile_bs ttl_exp68 ttl_exp69
```/* End */

from the previous message.

T

On Wed, Jun 10, 2009 at 3:57 PM, Susanne
Neckermann<sneckermann@iew.uzh.ch> wrote:
```
```Hi!

I want to test whether the difference in the fifth percentile of two
distributions is significant.
```
In principle, this test should be similar to the "median" (thats the name of the command) test already implemented in Stata, but unfortunately I cannot
```program .ado files and I could not find a user-written procedure.
P.S. the number of observations in the fifth percentile is pretty low
(around 6), so one might have to use something with bootstrap or
permutations.

Thank you so much in advance
susanne

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```
```

--
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```
```

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```