Jan Bayer
>
> We have repeatedly determined a whole bunch of "phenotypic
> measures" on
> a group of individuals, over a period of three years now. I
> would like
> to use Stata to perform analyses of the data, but I'm a real newbee.
>
> In my datalist I have one line per subject (individual) for
> every single
> time the individual was tested. In columns, I have the
> numerical values
> for the different phenotypic measures. The same subject
> (individual)
> does show up several times in the datalist (several lines), and
> therefore, for every subject we have multiple independent
> determinations
> of the same phenotypic values.
>
> Is it possible to generate new variables containing the
> variance of the
> measurements, per subject (individual) ?
>
See help on -egen- which provides a canned -sd()- function.
So you could just
. bysort id : egen sdpheno = sd(pheno)
. gen varpheno = sdpheno^2
But better is to have your own function.
Typing
. which _gsd
shows you where the defining file is on your system.
Here is a copy.
*! version 3.1.0 30jun1998
program define _gsd
version 6
syntax newvarname =/exp [if] [in] [, BY(varlist)]
tempvar touse mean
quietly {
gen byte `touse'=1 `if' `in'
sort `touse' `by'
by `touse' `by': gen double `mean' = /*
*/ sum(`exp')/sum((`exp')!=.) if `touse'==1
by `touse' `by': gen `typlist' `varlist' = /*
*/ sqrt(sum(((`exp')-`mean'[_N])^2)/(sum((`exp')!=.)-1)) /*
*/ if `touse'==1 & sum(`exp'!=.)
by `touse' `by': replace `varlist' = `varlist'[_N]
}
end
The program for a variance could just be therefore
================================ begin _gvar.ado
*! _gvar NJC 17 Dec 2002
*! _gsd 3.1.0 30jun1998
program define _gvar
version 6
syntax newvarname =/exp [if] [in] [, BY(varlist)]
tempvar touse mean
quietly {
gen byte `touse'=1 `if' `in'
sort `touse' `by'
by `touse' `by': gen double `mean' = /*
*/ sum(`exp')/sum((`exp')!=.) if `touse'==1
by `touse' `by': gen `typlist' `varlist' = /*
*/ sum(((`exp')-`mean'[_N])^2)/(sum((`exp')!=.)-1) /*
*/ if `touse'==1 & sum(`exp'!=.)
by `touse' `by': replace `varlist' = `varlist'[_N]
}
end
============================== end _gvar.ado
What to do:
1. Copy and paste the code above into a text file
_gvar.ado in what your Stata calls STBPLUS.
Type
. sysdir
to see where this ado. The underscore _ is essential.
2. Within Stata, your commands are then of the form
. bysort id : egen varpheno = var(pheno)
Nick
[email protected]
*
* For searches and help try:
* http://www.stata.com/support/faqs/res/findit.html
* http://www.stata.com/support/statalist/faq
* http://www.ats.ucla.edu/stat/stata/
