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st: Bias in a zero truncated poisson


From   Laurie Molina <molinalaurie@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   st: Bias in a zero truncated poisson
Date   Sun, 31 Oct 2010 10:11:35 -0600

Hi all,
My dependent variable is a count data variable that takes only positive values.

I am running:
***
ztp depvar indepvars
predict number

sum depvar
sum number
****
but the mean of depvar is not the same as the mean of number.

On the other hand if i run
******
poisson depvar indepvars
predict number2

sum depvar
sum number2
******

the mean of depvar is the same as the mean of number2.

Why is this happening?
Aren't ztp estimates unbiased estimates?


Thank you all as always...

LM
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