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Re: st: RE: Create variable with historic means


From   "Alvarez,Sergio" <[email protected]>
To   <[email protected]>
Subject   Re: st: RE: Create variable with historic means
Date   Tue, 16 Aug 2011 14:34:55 -0400

Thanks Nick!  Works like a charm.

Sergio


On Tue, 16 Aug 2011 18:05:26 +0100, Nick Cox wrote:
Your bottom line implies surprise, but in general -if- automatically
ignores excluded observations, both in using data and in assigning
results.

Also, at first sight -egen- is fairly hopeless for problems that
somewhere involve leads or lags. But it may be that this will do what
you want.

by zone wave mode, sort: egen hckr = mean(catch/(year<2006))

There are two tricks here, one standard and one non-standard.

1. -egen, mean()- will work on expressions, which can be (much) more
complicated than single variable names.

2. The expression here is

catch / (year < 2006)

The Boolean (year < 2006) evaluates as 1 or 0, so the whole evaluates as

catch if year < 2006
.     if year == 2006

So the effect is to ignore 2006, but nevertheless to assign the
result to all years including 2006.

Another way to do it is

by zone wave mode, sort: egen hckr = mean(cond(year<2006), catch, .)

but I am fond of the first way.

For a write-up, see

SJ-11-2 dm0055  . . . . . . . . . . . . . .  Speaking Stata: Compared
with ...
        . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
N. J. Cox
        Q2/11   SJ 11(2):305--314                                (no
commands)
        reviews techniques for relating values to values in other
        observations

Nick
[email protected]

Sergio

I'm trying to create a new variable that contains the historical means of CATCH, but only using the previous years. My dataset looks something like
this:

YEAR   CATCH    ZONE    WAVE    MODE
2004       2           1           1          1
2005       3           2           3          2
...          ...          ...          ...        ...
2006

In this case I want to create a new variable that will take the value of the mean catch for the previous years (2004 and 2005) by ZONE, WAVE and MODE, but not use any of the 2006 observations to calculate the means. Its OK if the variable is created for the previous years as well, as I plan to drop
those observations once I can get the new variable.

I have been able to create a table that gives me the mean of CATCH by ZONE,
WAVE and MODE using:

table zone wave mode if (year<2006), contents (m catch)

And I can create a variable that has the means, using 2006 data, with:

by zone wave mode, sort: egen hckr = mean(catch)

When I try to create the variable using only the previous years, I have
used:

by zone wave mode, sort: egen hckr = mean(catch) if(year<2006)

But it just gives me missing values for all observations from year 2006.

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--
Sergio Alvarez
Food and Resource Economics
University of Florida
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