Re: st: Drop variables satisfying a condition

[David Kantor <kantor.d@att.net>]

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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