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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... * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

**Follow-Ups**:**Re: st: _N in by-groups***From:*Nick Cox <njcoxstata@gmail.com>

**Re: st: _N in by-groups***From:*Maarten Buis <maartenlbuis@gmail.com>

**References**:**st: _N in by-groups***From:*Matthew White <mwhite@poverty-action.org>

**Re: st: _N in by-groups***From:*Maarten Buis <maartenlbuis@gmail.com>

**Re: st: _N in by-groups***From:*Matthew White <mwhite@poverty-action.org>

**Re: st: _N in by-groups***From:*Nick Cox <njcoxstata@gmail.com>

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