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# Re: st: Re: Test for statistically significant difference in Poisson coefficients or predicted number of events

 From Austin Nichols To statalist@hsphsun2.harvard.edu Subject Re: st: Re: Test for statistically significant difference in Poisson coefficients or predicted number of events Date Wed, 6 Apr 2011 11:11:17 -0400

```Tony Love <tonyplove@gmail.com>:
The tests you want are in the regression output--a test that the
cond==2 group differs from cond==1 group is the test that the coef on
cond==2 is zero, which has a Z of 3.37and a p-value of 0.001, so those
groups differ significantly, and the cond==3 group does not differ
significantly from the cond==1 group (p-value of .357).

On Tue, Apr 5, 2011 at 9:09 PM, Tony Love <tonyplove@gmail.com> wrote:
>> Hi, I am working on a project for my advisor, so please don't crucify me for missing some elementary concept.   I understand most of the elementary concepts regarding Poisson in Stata, but I have what I think is a question of higher difficulty.
>>
>> I have been working with the use of factor variables, for example, estimating a Poisson model using a Stata command like 'poisson num_awards i.cond math'.  This would return the expected difference in log count between each condition and the reference group (cond=1).
>> The output might look something like this if there were 3 conditions:
>>
>> poisson num_awards i.cond math, vce(robust)
<snip>
>> My question is this:  Are you aware of a test to determine whether the differences in either the coefficients produced in the first example or the predicted number of events in the second example are statistically significantly different from one another?  That is, if I compare the coef for cond1 to cond2 and cond1 to cond3 and cond2 to cond3 OR predicted number of events for cond1 to cond2 and cond1 to cond3 and cond2 to cond3, are those differences statistically significant from one another, and if not, which ones are not?

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