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From |
James McJoseph <james.mcjoseph@gmail.com> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: st: ZIP with Sample Selection |

Date |
Wed, 1 Dec 2010 11:24:29 +0200 |

Thanks, for this Maarten. I will sleep over it. On Wed, Dec 1, 2010 at 10:46 AM, Maarten buis <maartenbuis@yahoo.co.uk> wrote: > --- On Wed, 1/12/10, James McJoseph wrote: >> 2. Is it appropriate to apply a count model to a data where >> the only count present in the data are: 0, 1, 2. Counts above >> 2 are possible but very extremely rare. Another consideration, the >> occurance of counts are not equally probable. 0 is more probable >> than 1. 1 is more probable than 2, Hence 3 and above are much less >> probably to even unlikely. > > Sounds consistent with a Poisson distribution with a extrememly low > mean. One problem I can imagine is that in your subject line you > refer to a zero inflated poisson. A regular Poisson with such an > extremely low mean will have a huge amount of zeros. I can imagine > that Stata will have trouble distinguishing between a zero inflated > Poisson distribution and such an extreme Poisson. Add a selection > equation to that, and I doubt whether you'll ever get convergence. > Think of it this way: what we do when we do a statistical analysis > is to compare people (or whatever your unit of analysis may be). If > everybody where the same, such a comparsion would be very boring. > In other words it is the differences between people that contains > the information we use in our analysis. If you have such an extreme > Poisson distribution, then your variance is likely to be very low, > which means there is very little information present in your data. > A consequence of that is that even if the dataset is large, you can > still only estimate relatively simple models and certainly not a ZIP > with sample selection, as both the ZIP part and the sample selection > part require lots of information on their own, let alone in > combination. > > 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/ > * * 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**:**st: ZIP with Sample Selection***From:*James McJoseph <james.mcjoseph@gmail.com>

**Re: st: ZIP with Sample Selection***From:*Maarten buis <maartenbuis@yahoo.co.uk>

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