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st: retry: copy value withing group
My appologies. The first several lines of my previous e-mail were
mysteriously deleted. Hopefully this one is complete.
*Intro*: Here's a simple Stata task I solved years ago and do
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
*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 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.
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