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From |
"Naji Nassar (MIReS)" <naji.nassar@mires.fr> |

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

Subject |
st: RE: Two Part Model |

Date |
Fri, 25 Jun 2004 17:18:39 +0200 |

Simone, I used to model a twopart model in marketing (Buy/NoBuy as cloglog, and quantity as truncated Poisson) The likehood for each observation P(y=0)=1-p(y>0) P(y)=P(y>0).P(y|y>0) I think you'll have to take into account all the observations (even those who never incur an expense).. Please pay attention to the percentage of 'patients' (if less than 5% you'll get rare events biases..) Normally, as the 'non-patients' show no variance for the expenses, the two models show no significat correlations You can also try a two step modelling (one estimation for the decision -all observations-, an other for the expenses -patients-..) Hope this helps Naji -----Message d'origine----- De : owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu]De la part de Simone Boyce Envoye : vendredi 25 juin 2004 17:01 A : Stata Objet : st: Two Part Model Hi, I'm interested in modeling a two part model, which seems to go by many names in different fields: two-tiered and hurdle models. Essentially, I'm trying to model medical cost expenditure, but the data consist of many individuals who never incur an expense. Hence, there are many zeroes in the data. As a result, I'm separating the decision into two parts: 1) the decision to see a doctor and 2) the amount of the expense: P(y>0) =X_1B_1+e_1 (1) E(y|y>0) = log(y)|y>0= X_2B_2+e_2 (2) where the error terms between equations (1) and (2) are independent. I know that this is usually estimated as a probit (eqn 1) and then OLS (eqn 2) to compute E(y)= P(y>0)E(y|y>0). However, how do I determine the marginal effects and the associated standard errors? Especially, if one of the variables that I'm interested in is in both equations. Has anyone programmed the two part model? Thanks, Simone __________________________________ Do you Yahoo!? New and Improved Yahoo! Mail - Send 10MB messages! http://promotions.yahoo.com/new_mail * * For searches and help try: * http://www.stata.com/support/faqs/res/findit.html * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/ * * For searches and help try: * http://www.stata.com/support/faqs/res/findit.html * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

**References**:**st: Two Part Model***From:*Simone Boyce <simonepeart@yahoo.com>

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