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Re: st: Drop variables satisfying a condition


From   "Austin Nichols" <austinnichols@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Drop variables satisfying a condition
Date   Fri, 6 Jul 2007 13:03:19 -0400

David Kantor et al.:
I agree that testing sum=0 is inappropriate, and there is no need to
invoke -summarize- and all its machinery when -count- will do; but I
think David Kantor does not mean
capture confirm `var' == 0
but rather
capture assert `var' == 0
as the first will always fail.

Given that Maarten and I have both posted programs to handle the
string variables, David Elliot's program need not be fixed, I think.

On 7/6/07, David Kantor <kantor.d@att.net> wrote:
David Elliot wrote in response to the question from Guillermo Villa:

[...]
>        quietly summarize `var'
>        if `r(sum)' == 0  { // r(sum) is returned by summarize
>             drop `var'
>             display as txt "Dropping " as result "`var'"
>            }
[...]

I don't think that testing `r(sum)' == 0 is appropriate. This
indicates that the sum of the values is 0. I think you really want to
test whether all the values are 0. And it's easy to construct an
example where the sum is 0 but not all values are 0.

I would suggest...

capture confirm `var' == 0
if ~_rc {
  drop `var'
}

Another thing you may want to test is whether r(sd) ==0 after a
-summarize-, which tells you that the values are constant.

Also note that the quotes around r(sum) were unnecessary, though that
is a moot point.

HTH
--David

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