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st: AW: What's the added value of having -in- subset the data before -if- does?


From   Dan Blanchette <dan.blanchette@duke.edu>
To   statalist@hsphsun2.harvard.edu
Subject   st: AW: What's the added value of having -in- subset the data before -if- does?
Date   Wed, 4 Feb 2009 12:17:44 -0500 (EST)

The downside to that solution:
. bysort foreign: list  if foreign & _n <= 10
is that the -bysort- will change the sort order of the dataset (in other datasets other than the auto.dta data since the auto data just happens to be sorted by foreign. It also might be the case that the condition you are interested in is not sort order related. This will list up to 10 obs per value of mpg that is less than 20:

. bysort mpg: list  if mpg < 20 & _n < 10

Also, I realized that since sum() creates a running
sum this will not just list the first 10 observations
because 0 (false) is also less than 10.  So this:

. list  if sum((foreign == 1)) <= 10

should be:

. list  if sum((foreign == 1)) <= 10 & foreign == 1

or:

. list  if inrange(sum((foreign == 1)),2,11) & foreign == 1


A better example to show how the running sum also requires
the condition to be repeated would be:

//not correct because it will list observations after the // first instance where (mpg < 20) until the sum reaches 10
. list  if inrange(sum((mpg < 23)),1,10)

//correct
. list  if inrange(sum((mpg < 20)),1,10) & mpg < 20

So you have to repeat the condition to subset the dataset to just those observations before starting the running sum.

Yet another learning experience.

Dan Blanchette
Research Associate
Center of Entrepreneurship and Innovation
Duke University's Fuqua School of Business
Durham, NC USA
dan.blanchette@duke.edu


From 	  "Martin Weiss" <martin.weiss1@gmx.de>
To 	  <statalist@hsphsun2.harvard.edu>
Subject 	  st: AW: What's the added value of having -in- subset the data before -if- does?
Date 	  Wed, 4 Feb 2009 17:01:33 +0100

<>


*************
bys for: list  if foreign &_n<=10
*************



HTH
Martin

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