st: Counting Unique Values by Year

["Jennifer S. Earl" <jearl@soc.ucsb.edu>]

I have a data set with cases spread out over a number of years. I have a 
numeric variable called CLMS. I want to create a new variable UNIQCLMS that 
equals the number of unique values that CLMS took on each year.

I have thought of some very long-winded ways to do this, such as creating a 
counter using a lag-comparison and then harvesting the last value of this 
counter, but it seems like it should be easier. In particular, Stata 
already calculates the number of unique values in lots of operations, 
including INSPECT (e.g., "by year: inspect clms" will produce the number of 
unique values for CLMS, unless that number exceeds 99, but it won't write 
that value out to another variable as far as I know), and the number of 
unique values should also equal the number of rows produced using "by year: 
tab clms".

So, I am hoping someone might be able to think of a quick and/or elegant 
way to get Stata to produce a new variable, UNIQCLMS that contains the 
number of unique values that CLMS takes on in each year. If I could dream 
up a new egen command, the format would be something like:

by year: egen uniqclm=unique(CLMS)

Any thoughts would be appreciated.

Cheers,

Jennifer

*********************************
Jennifer Earl
Assistant Professor
Department of Sociology
University of California
Santa Barbara, CA  93106-9430

P: (805) 893-7471
F: (805) 893-3324
*********************************

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