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From |
Maarten buis <maartenbuis@yahoo.co.uk> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: st: Heckprob - Why no margins in selection equation? |

Date |
Tue, 30 Nov 2010 13:27:49 +0000 (GMT) |

--- On Tue, 30/11/10, James McJoseph wrote: > Could someone enlighten me plse. I use stata 11. I > estimated -heckprob-. I requested -margins, dydx(_all)-. > Stata gives me margins for the obejective equation > only. For the selection equation, it gives "missing". > Why? A marginal effect is the first derivative of some outcome measure with respect to explanatory variables. In models like -heckprob- there are multiple outcome measures posible: the marginal probability of success, the probability of success given that someone is selected, the probability of being selected, etc. So -margins- needs to know what outcome measure you mean. By default, it looks at the predict option of your command, in this case -heckprob-, and looks at the default prediction and assumes that that is what you want. You can (and should) find out yourself what that default is by typing in Stata -help heckprob postestimation-. Then you will find that the default prediction of -heckprob- is the marginal probability of succes, so it makes no sense to have that marginal effect type for the selection equation and Stata wisely keeps them missing. You can change your outcome measure by specifying the -predict- option in -margins-. Hope this helps, Maarten -------------------------- Maarten L. Buis Institut fuer Soziologie Universitaet Tuebingen Wilhelmstrasse 36 72074 Tuebingen Germany http://www.maartenbuis.nl -------------------------- * * 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: Heckprob - Why no margins in selection equation?***From:*James McJoseph <james.mcjoseph@gmail.com>

**References**:**st: Heckprob - Why no margins in selection equation?***From:*James McJoseph <james.mcjoseph@gmail.com>

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