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st: copy value within group


From   frakt@bu.edu
To   statalist@hsphsun2.harvard.edu
Subject   st: copy value within group
Date   Mon, 23 Oct 2006 09:34:40 -0400

frequently, yet I have never been happy about how I do it.  I've always
wondered if there was another way.

*Fact*: In my data there exists a variable, x, with the following
property (known by construction or verified with an -assert-).  Within
each group of observations identified by a unique id (call the variable
id), x is missing for all but one observation.

*Operation*: I want to assign the single nonmissing value of x to all
missing values of x within the group.  That is, when I'm done, x is
nonmissing for all observations and constant within id.  This  constant
value of x (within id) remains unchanged for the original nonmissing
values of x.

*My Solution*: Usually I accomplish this with something like:

bysort id: egen t = mean(x) //other -egen- commands work, like -max()-//
replace x = t drop t

*Comment*: This works because I choose an -egen- command that ignores
missing values.  What displeases me about this is that when I review my
code later, it is hard to tell what I'm really doing since my motivation
is not to compute a mean or max, per se, but just to execute
a sort of copy operation within group.  What would be nice is if there
were a command named copy() (or something clear) that (1) verified the
properties I require (see "Fact" above) and then (2) did the operation
I want (see "Operation" above).  Even a more transparent way of just doing
(2) (and not (1)) would be an improvement.  I'm not skilled enough to
easily write such a command.  Perhaps there is such a command or some
other way to do what I want that is more transparent than my solution.
Any suggestions?

Thanks.




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