Re: st: Carry over information on time-invariant covariate to all observations of a household?

[David Kantor <kantor.d@att.net>]

At 04:28 AM 8/3/2010, Jen Zhen wrote:
> Dear Listers,
>
> suppose I have a panel with dimensions Household and Year. I have the
> time-invariant household characteristic X. Information on it is
> currently given for each household in only one of the years, but I
> would like to carry over this information to all observations of each
> household.
>
> To illustrate, the dataset looks like this:
>
> HH   Year    X
> 1     1990     5
> 1     1991     .
> 1     1992     .
> 2     1990     .
> 2     1991     3
> 2     1992     .
> 3     1990     .
> 3     1991     .
> 3     1992     2
>
> and I would like to fill in the missing values.
>
> Currently my way of doing it is this:
> - bysort HH: egen X2 = max(X) -
> - replace X = X2 -
>
> However, in a large dataset running this command takes forever, so I
> am wondering whether there is a faster way to do this?

In addition to other advice given, you may want to look into 
-carryforward- on SSC.
ssc desc carryforward
ssc inst carryforward

It will carry forward the latest value in the sort order (presumably 
by HH and some other variable), whereas the advice given by Martin 
Weiss will carry the lowest value in each group.
I noticed that the one observation (per HH group) that has a value is 
not always the first. Your method needs to address whether you want 
that value spread in both directions or just forward.

If you do use -carryforward-, you will need to decide on the sort 
order, which might be HH X or HH Year. If it is HH X, it will carry 
the greatest value forward, which is the same as the lowest if there 
is only one observation per HH group having a value in X. If you use 
HH Year, then it will carry values only into later years only -- not 
into earlier years. If you want the value spread backwards as well, 
you can follow it with a backward -carryforward-:
bysort HH (Year): carryforward X ...
gen int negyear = -year
bysort HH (negyear): carryforward X ...

This may be a more generally correct method, if there are any HHs 
with more than one value for X.
But if you are certain that all HHs have only one value for X, then 
the advice in Martin Weiss's first reply is correct and simplest.

HTH
--David

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