Re: st: RE: programatically dropping variables that don't actually vary

[Nick Cox <njcoxstata@gmail.com>]

In principle, many variables could have mean 0. A safer test is that

if r(min) == 0 and r(max) == 0

Nick

On Thu, Aug 9, 2012 at 8:03 PM, Sarah Edgington <sedging@ucla.edu> wrote:
> Jenn,
> There are a variety of ways you might want to do this.  What I would do is
> something like the following:
>
> foreach var of varlist dummy1-dummyn {
>         sum `var',  meanonly
>         if r(mean)==0 {
>                 drop `var'
>         }
> }
>
> This cycles through each of your variables (substitute your actual variable
> list for "dummy1-dummyn").  For reach variable it calculates the mean.  The
> drop statement in the if loop only gets executed if the value stored in
> r(mean) is 0.
>
> -Sarah
>
>
> -----Original Message-----
> From: owner-statalist@hsphsun2.harvard.edu
> [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Earl, Jennifer
> Suzanne - (jenniferearl)
> Sent: Thursday, August 09, 2012 11:46 AM
> To: statalist@hsphsun2.harvard.edu
> Subject: st: programatically dropping variables that don't actually vary
>
> Hi,
>
> I am working with a large number of dummy variables and using collapse to
> create derivative datasets that are the frequencies of 1's for each dummy
> variable (a couple of hundred through foreach loops). I want to drop any of
> the dummy variables that never had a 1 (so mean(dummy1)==0, or
> max(dummy)==0) but it seems that drop only lets you use an if statement to
> drop observations, but not an if statement to drop variables.
>
> My best guess is to use a list means to create a list of the variable names
> that can be stored in a local and then fed into a drop command, but can't
> seem to make that work either since I only want the list of variable names
> that have a mean of 0. Or maybe transpose the dataset, drop then since the
> variables are now observations, and transpose back? Another solution would
> be save through StatTansfer and use it's drop constants feature, and then
> bring the data back in, but there must be an easier way.
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