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From |
"Martin Weiss" <martin.weiss1@gmx.de> |

To |
<statalist@hsphsun2.harvard.edu> |

Subject |
RE: st: RE: RE: what does _N mean under by varlist in the -egen- and -gen-? |

Date |
Sat, 8 Aug 2009 22:20:41 +0200 |

<> Well, you used -gen- twice and -egen- twice, -gen- behaved according to your expectation twice. -egen- behaved as expected when fed an -expression- involving _N, but failed you when you used the same -expression- in an -if- qualifier statement. I am reluctant to call this a rule, but this is where our little investigation stands. The entry in the data management manual ("[D]") seems to confirm this suspicion. As I showed in one of my posts today, it is easy to get what you want using an -expression- like - ((sex[_N]==0)*sex)-. The part in the round parantheses evaluates to one for all the observations in families that have sex equal to zero for their last observation, so any summation over this -expression- sums only the ones where the condition holds. BTW, you do not specify your sort order within families, so should anything happen to it along the way, the "_N"- observation would also change... HTH Martin -----Original Message----- From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of gjhxmu@sina.com Sent: Samstag, 8. August 2009 19:54 To: statalist Subject: Re: st: RE: RE: what does _N mean under by varlist in the -egen- and -gen-? Martin,thank you for your help! In -egen-, in my example sometimes _N means the total dataset, while sometimes means not. Is there a rule? btw, where to see " Explicit subscripting (using _N and _n), which is commonly used with generate, should not be used with egen;" See [D], p. 145? What is [D], p. 145? Thank you very much! Best regards, Rose. bys family: g x=sum(sex) if sex[_N]==0 bys family: egen xx=total(sex) if sex[_N]==0 bys family: g xxx=sex[_N] bys family: egen xxxx=total(sex[_N]==0) l,sepby(family) noobs ----- Original Message ----- From: Martin Weiss <martin.weiss1@gmx.de> To: <statalist@hsphsun2.harvard.edu> Subject: st: RE: RE: what does _N mean under by varlist in the -egen- and -gen-? Date: 2009-8-8 21:56:34 <> The manuals make the problem clear, though: " Explicit subscripting (using _N and _n), which is commonly used with generate, should not be used with egen;" See [D], p. 145 - di in smcl " {manpage D 153: Click here!}"- HTH Martin -----Original Message----- From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Martin Weiss Sent: Samstag, 8. August 2009 15:30 To: statalist@hsphsun2.harvard.edu Subject: st: RE: what does _N mean under by varlist in the -egen- and -gen-? <> You can of course form an expression that does yield the correct result: *** clear input family sex 1 1 1 1 1 1 2 0 2 1 2 0 2 1 2 0 3 0 3 1 3 0 3 1 3 1 3 1 end //complex expression fed to -total- bys family: egen x=/* */total(sex*(sex[_N]==0)) //compare with -if- solution bys family: egen xx=total(sex) /* */ if sex[_N]==0 list, noobs sepby(fam) *** HTH Martin -----Original Message----- From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of gjhxmu@sina.com Sent: Samstag, 8. August 2009 11:40 To: statalist Subject: st: what does _N mean under by varlist in the -egen- and -gen-? Dear statalists, I know that under by varlist: _N is interpreted within each group of observations, not for the whole dataset. However, I found the meaning _N in the -egen- is a little different. Take the following for example, clear input family sex 1 1 1 1 1 1 2 0 2 1 2 0 2 1 2 0 3 0 3 1 3 0 3 1 3 1 3 1 end bys family: g x=sum(sex) if sex[_N]==0 bys family: egen xx=total(sex) if sex[_N]==0 bys family: g xxx=sex[_N] bys family: egen xxxx=total(sex[_N]==0) l,sepby(family) noobs +------------------------------------+ | family sex x xx xxx xxxx | |------------------------------------| | 1 1 . . 1 0 | | 1 1 . . 1 0 | | 1 1 . . 1 0 | |------------------------------------| | 2 0 0 . 0 5 | | 2 1 1 . 0 5 | | 2 0 1 . 0 5 | | 2 1 2 . 0 5 | | 2 0 2 . 0 5 | |------------------------------------| | 3 0 . . 1 0 | | 3 1 . . 1 0 | | 3 0 . . 1 0 | | 3 1 . . 1 0 | | 3 1 . . 1 0 | | 3 1 . . 1 0 | +------------------------------------+ Based on the results, Obviously the meaning of _N in the first two is different, while the meaning of _N in the last two is the same. Could anyone tell me why? Thank you very much. Best regards, Rose. * * 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/ * * 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/ * * 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/ * * 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/ * * 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/

**References**:**Re: st: RE: RE: what does _N mean under by varlist in the -egen- and -gen-?***From:*gjhxmu@sina.com

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