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Re: st: _N in by-groups


From   Nick Cox <njcoxstata@gmail.com>
To   "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>
Subject   Re: st: _N in by-groups
Date   Fri, 19 Aug 2011 16:58:34 +0100

I agree with Maarten that this is generally a bad idea.

Somewhere in the documentation there is a warning about trying to use _n with -egen-. For example, -egen- often works temporarily with a different sort order.

One way to get what you want is a two-step

egen max = max(var1), by(group)

egen mean = mean(var1/(var1 < max)), by(group)

The -if- way is often not what you want here, as discussed recently on the list.

Nick

On 19 Aug 2011, at 15:40, mcross@exemail.com.au wrote:

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 :- pref ix
(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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