Joel Clovis
> I want to delete variables if they do not contain enough
> observatoins. I've
> tried to make use of Nick Coxs' npresent & dropmiss (STB-49:dm67 &
> STB-60: dm89) to no avail, some attempts are in the
> following manner:
>
> foreach var of varlist cba2tfina-region {
> keep `var' if r(npresent)>=30
> }
>
> this brings up invalid syntax, and I'm not sure why and
>
> keep if npresent(lifeinsdensity )<30
>
> results in 0 observations being deleted, even though
> npresent(lifeinsdensity ) without keep..returns lifeinsdensity 10
>
> Can anyone help? I'm a Stata 7 user
1. Use of -npresent-
====================
-npresent- is not a function, so -npresent()- is not
legal within a command as in your example with -keep-.
. npresent (lifeinsdensity)
does work, but this is a gift from Stata's varlist
syntax, and nothing to do with functions.
-npresent- does leave behind r(N), but not r(npresent).
This is a side-effect of its use of -count-.
-npresent- is really a function to display information.
I wouldn't use it this way.
2. How to -drop- according to your criterion
============================================
I'd do it this way:
. foreach v of var cba2tfina-region {
. qui count if !missing(`v')
. if r(N) < 30 {
. drop `v'
. }
. }
You need to be straight on the difference between (say)
if r(N) < 30 {
drop `v'
}
and
drop `v' if r(N) < 30
As it happens, the second is not legal syntax,
but the difference in general is important:
see http://www.stata.com/support/faqs/lang/ifqualifier.html
3. -dropmiss-
=============
As far as I can see, -dropmiss- offers nothing
for this problem over the first principles approach.
Nick
[email protected]
*
* For searches and help try:
* http://www.stata.com/support/faqs/res/findit.html
* http://www.stata.com/support/statalist/faq
* http://www.ats.ucla.edu/stat/stata/
