Re: st: RE: Create variable with historic means

["Alvarez,Sergio" <sergioal@ufl.edu>]

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
> n.j.cox@durham.ac.uk
>
> 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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