
From  Scott Cunningham <scunning@gmail.com> 
To  statalist@hsphsun2.harvard.edu 
Subject  Re: st: interpreting marginal effects of poisson 
Date  Wed, 19 Apr 2006 09:21:05 0400 
Description of Data:
Question: The coefficient on sr is .6870002, and it is the variable of interest. Am I correct in the following interpretations:
Q1. The constant (_cons) represents the log of the mean number of sex partners for the reference cell, which in this case is Black men living in homes where parents are not married (hhd1=1 if biological parents are married). Since exp{.5387156)=1.7138, we see that on the average these men have 1.7 sex partners at this point in their lives. Is this the correct interpretation?
Q2. The hhd1 variable reflects the state of the family in which the Black male lives. As we move from hhd1=0 to hhd1=1, the log of the mean decreases by .7, which means that the number of sex partners gets multiplied by exp{0.300639)=0.740345. This means that Black males with married biological parents have 25% fewer sex partners than their counterparts whose parents are not married. Is this the correct interpretation?
Q3. The sr variable is continuous. It is the ratio of eligible Black males (of a certain age range) to eligible Black females (of like age range) at the state level, and will take on a value from as low as 0.3 to 2.5. I am unsure of how to interpret the marginal effect of a change in the sex ratio ("sr") on sex partners. Am I correct that a one unit increase in the sex ratio causes recent sex partners for Black males to fall by 50%? If so, what is a "one unit" when we are talking about a continuous random variable? Is it a one unit increase in the standard deviation? I've been unable to find this information from my reference books, and do not currently own the Stata book on categorical variables. But if anyone can provide basic help here, I'd appreciate it.
Q4: I am able to estimate a model with state, year and individual fixed effects using OLS with FE (xtreg), but not Poisson ( xtpoisson nor poisson). Specifically, I can estimate the Poisson model with year and individual effects, but not with year, state and individual fixed effects. When I include the state effects, the likelihood iteratations hang up, even after letting it go for thousands of iterations. It has hung up on a single likelihood, for that matter, and does not appear to be moving closer towards convergence. What could be causing this?
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