Re: st: _N in by-groups

[Nick Cox <njcoxstata@gmail.com>]

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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