# Re: st: RE: Outlier: Detection

 From "Austin Nichols" To statalist@hsphsun2.harvard.edu Subject Re: st: RE: Outlier: Detection Date Wed, 20 Feb 2008 11:48:18 -0500

```Sergiy, is there a reason to limit n to 90, or to use -inspect-
(necessarily limiting n to 99)?  Would this version accomplish the
same goal?

program Grubbs2, rclass sortpreserve
syntax [varlist] [if] [in] [, Level(int 95)]
marksample touse
foreach v of local varlist {
tempvar c
qui bys `v' `touse': g `c'=_N-_n if `touse'
qui count if `c'==0 & `touse'
local n=r(N)
local t2=(invttail(`n'-2,(1-`level'/200)/(2*`n')))^2
local G_cr=((`n'-1)/sqrt(`n'))*sqrt(`t2'/(`n'-2+`t2'))
quietly levelsof `v' if `touse', local(levs)
if `: word count `levs''!=`n' error 198
loc levsum=0
loc sqsum=0
foreach lev of local levs {
local levsum=`levsum'+`lev'
local sqsum=`sqsum'+`lev'*`lev'
}
local mean=`levsum'/`n'
local levsdev=sqrt(`sqsum'/`n'-`mean'*`mean')
local outliers
foreach lev of local levs {
local Z=abs(`mean'-`lev')/`levsdev'
if `Z'>`G_cr' local outliers "`outliers' `lev'"
}
di as txt "Outliers in `v': " as res "`outliers'"
}
return local outliers="`outliers'"
end

sysuse auto
Grubbs2 pr-gear, lev(99)
Grubbs2 pr-gear if for==1, lev(99)
Grubbs2 pr-gear if for==0, lev(99)

(Disclaimer: I have not read the Grubbs article, but I share Maarten's
skepticism about the utility of this approach.)
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```