# Re: st: creating average over past three years within groups

 From "Fabian Brenner" To statalist@hsphsun2.harvard.edu Subject Re: st: creating average over past three years within groups Date Fri, 03 Oct 2008 16:32:52 +0200

```wow! Thank you very very much. And thanks to Martin, as well. Both solutions work. Great help!

Fabian

-------- Original-Nachricht --------
> Datum: Fri, 3 Oct 2008 14:26:36 +0100
> Von: "Eva Poen" <eva.poen@gmail.com>
> An: Statalist <statalist@hsphsun2.harvard.edu>
> Betreff: Re: st: creating average over past three years within groups

> Fabian,
>
> the error message you get relates to the -local- statements. -local-
> does not allow -if-. A local macro acts like a placeholder and not
> like a variable.
>
> Take a look at your code:
>
> local median3 = oneyearmedian if year == `= `y' - 3' & industry == `k',
>
> First, you don't need the `=`y'-3' construct here, and the comma at
> the end is illegal. More importantly, you ask Stata to perform
> something in one step for which it needs two steps:
> - find out what the median is for industry k three years ago,
> - store this in a local macro.
>
> You can do it like this:
>
> sum oneyearmedian if year==`y'-3 & industry == `k'
> local median3 = r(mean)
>
>
> However, you could get all of this a lot easier by calculating the
> three year average median for one observation per year and industry,
> and then propagating it along. I assume you have some panel identifier
> called id.
>
> sort year industry id
> by year industry: gen first = _n==1
>
> sort first industry year
> by first industry: gen threeyearmean = (oneyearmedian[_n-1] +
> oneyearmedian[_n-2] + oneyearmedian[_n-3])/3 if first==1
>
> bysort year industry (first) : replace threeyearmean = threeyearmean[_N]
>
>
> (Untested.)
>
> Hope this helps,
> Eva
>
>
> 2008/10/3 Fabian Brenner <Fabian.Brenner@gmx.de>:
> > Dear all:
> >
> > I used -bysort year group : egen oneyearmedian = median(age)- to create
> the median within each group (1-10) for each year (1974-2000).
> >
> > Now I want to get the three year average of the three median for each
> group for the most recent three years:
> >
> > generate threeyearaverage = .
> > sort year group
> >
> > quietly forvalues k = 1/10 {
> > quietly forvalues y = 1974/2000 {
> > local median3 = oneyearmedian if year == `= `y' - 3' & industry == `k',
> > local median2 = oneyearmedian if year == `= `y' - 2' & industry == `k',
> > local median1 = oneyearmedian if year == `= `y' - 1' & industry == `k',
> > replace fiveyearmedian = (`median3' + `median2' + `median1' ) / 3 if
> year == `y' & group == `k'
> > }
> > }
> >
> > But stata always says: if not allowed r(101);
> >
> > What do I wrong? Hope someone can help me.
> > Fabian Brenner
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