st: Re: RE: why the different?

["Michael Blasnik" <michael.blasnik@verizon.net>]

"Wanli Zhao" <zhaowl@temple.edu> wrote:

> I found something interesting & puzzling. Maybe I just miss something. I
> have a dataset like this:
<snip>
> Now, I want to have the number of people older than 68 by each gvkey. So I
> do {egen old=count(age) if age>=69,by(gvkey)}. Then I found that the 
> number
> is correct but it only shows when the age variable is 69 or bigger. I
> thought it would put the same number within gvkey for each age, just as I
> experienced a lot of such functions do. Certainly, I did the following:
> gsort gvkey -old
> by gvkey: replace old=old=[_n-1] if old==.
>
> That's OK. But for the outsider, I want the number of 1's within each 
> gvkey
> so I did {egen outside=sum(outsider), by(gvkey)}. This time, there is no
> missing value. Why the "count" behaves differently? Certainly, I can
> generate another dummy for age bigger than 68 and then sum that up. Same
> result. But I just wonder why "count" did not fill in all the values?
>
> Cheers,
> Wanli Zhao
I think this behavior can be frustrating at times, but it certainly isn't 
puzzling and I'd like to know what your examples are of other Stata commands 
that don't follow this convention.  Commands that use -if- clauses usually 
only operate on observations meeting the qualifier: gen x2=x^2 if x>5  will 
create missing values in x2 for any cases where x is not greater than 
.  -egen- follows this same behavior and your example with the egen sum 
doesn't have an -if- clause.  I have long thought that there ought to be an 
egen option for filling in these missing values when a function yields a 
constant for each by group.  Sometimes you can use logical conditions within 
the function to accomplish this, as in egen sumgt68=sum(x*(age>68)), 
by(gvkey).

Michael Blasnik
michael.blasnik@verizon.net

*
*   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/