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Fwd: Re: st: Collapsing Over Limited Set


From   Nick Winter <nw53@cornell.edu>
To   Stata List <statalist@hsphsun2.harvard.edu>
Subject   Fwd: Re: st: Collapsing Over Limited Set
Date   Thu, 15 Sep 2005 15:52:47 -0400

Sorry ... sent the last email by mistake. Trying again:

I believe this will do it:

bysort product: egen n_p = count( markup<. & company<. )
by product: egen sum_p = sum( markup * !mi(company) )

bysort company product : egen n_cp = count( markup<. )
by company product: egen sum_cp = sum(markup)

gen averagemarkup = (sum_p-sum_cp) / (n_p-n_cp)


If you know you don't have missing values on company or on markup, then you can simplify:

bysort product: gen n_p = _N
by product: egen sum_p = sum(markup)


bysort company product : gen n_cp = _N
by company product: egen sum_cp = sum(markup)

gen averagemarkup = (sum_p-sum_cp) / (n_p-n_cp)


That is, get the sum and number of observations for each product overall, then get the sum and number of observations for each company-product pair...then subtract off these latter quantities from the former to calculate the average you want.

Note in the first approach, "markup<." means "nonmissing on markup", but the egen function count() doesn't allow the missing() function to appear within it.



--Nick Winter







At 03:11 PM 9/15/2005, you wrote:
Hi there,

I would really appreciate someone's help with this question.

I'm trying to generate a dataset of statistics by collapsing another
dataset, but each of the id's I'm collapsing by, I want to use every
observation in the dataset except the one for the id under consideration.
Would there be a way to do this?

For example, I have data of the form:

company product markup
100     31      .3
100     55      .2
111     31      0
111     55      .1
120     31      .1
120     55      .1

Now I want to ask the question: for each company, calculate the average
markup of each product it produces, where the average is taken over all
companies that sell the product except the company itself. So I want to
end up with

company product averagemarkup
100     31      .05
100     55      .1
111     31      .2
111     55      .15
120     31      .15
120     55      .15

Obviously collapsing the data the standard way is not going to do this. I
need to do this for hundreds of thousands of observations (hundreds of
companies and thousands of products) so am looking for a way to do this
that would be relatively quick. I would be grateful for any suggestions.

Thanks very much.

Jason
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________________________________________________________
Nicholas J. G. Winter                     607.255.8819 t
Assistant Professor                       607.255.4530 f
Department of Government              nw53@cornell.edu e
Cornell University        falcon.arts.cornell.edu/nw53 w
308 White Hall
Ithaca, NY 14853-4601
________________________________________________________
Nicholas J. G. Winter 607.255.8819 t
Assistant Professor 607.255.4530 f
Department of Government nw53@cornell.edu e
Cornell University falcon.arts.cornell.edu/nw53 w
308 White Hall
Ithaca, NY 14853-4601

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