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Re: st: _N in by-groups


From   mcross@exemail.com.au
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: _N in by-groups
Date   Sat, 20 Aug 2011 00:40:45 +1000

I too am confused regarding when _N is or isn't influenced
by the ?by :- prefix.

I would like to remove a single outlier from each group within
the following data set...

input group var1

1      4
1      5
1     81
2      2
2      3
2      3
2     72

end

I would then like to calculate the mean for each group (with the outliers
gone).

I assumed that the following code would do the trick?


by group (var1), sort: egen average = mean(var1) if var1 != var1[_N]


When the mean was calculated ? it did so following the ?by :- prefix
(i.e. _N = 3 for group 1). But following the ?if- option, _N was
calculated from the whole data set (i.e. _N = 7).

I got around this problem by generating/sorting a byte tag, however, I still
don?t understand WHY and HOW Stata does this.

Could I have dealt with the above using a single line of code?

Cheers,

Mike (beginner Stata 8)




* So _N, as it were, never sees the -by:- and is not influenced
by it.

** If a Stata command has by-groups, it seems like _N is interpreted
sometimes as the number of observations in the by-group and sometimes
as the number of observations in the data set.

*** If you use the -by :- prefix it is always defined as the number of
observations within each by-group. Stata would be a pretty lousy
program if such a scalar randomly changed meaning...



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