question about poisson regression

Many thanks.
Another question:
model<- glm(count ~ drug*result, family = poisson)

anova(model,test="Chisq")
? ? ? ? ?Df Deviance Resid. Df Resid. Dev ?Pr(>Chi) ? ?
NULL ? ? ? ? ? ? ? ? ? ? ? ? ? ?3 ? ? 47.522 ? ? ? ? ? ? ?
drug ? ? ? ? 1 ? ?0.032 ? ? ? ? 2 ? ? 47.491 ? 0.85858 ? ?
result ? ? ? 1 ? 41.187 ? ? ? ? 1 ? ? ?6.304 1.383e-10 ***
drug:result ?1 ? ?6.304 ? ? ? ? 0 ? ? ?0.000 ? 0.01205 * ?

T! he pvalue of drug is?0.85858,which indicates that the total count of
drug1 and drug2 are not significantly different at 0.05 level.

But:

summary(model)
Coefficients:
? ? ? ? ? ? ? Estimate Std. Error z value Pr(>|z|) ? ?
(Intercept) ? ? 2.0794 ? ? 0.3536 ? 5.882 4.06e-09 ***
drug2 ? ? ? ? ? 0.9163 ? ? 0.4183 ? 2.190 ? 0.0285 * ?
result2 ? ? ? ? 1.9095 ? ? 0.3788 ? 5.041 4.64e-07 ***
drug2:result2 ?-1.1211 ? ? 0.4650 ?-2.411 ? 0.0159 * ?

The pvalue of drug2 is ?0.0285 * ?,which indicates that the total count of
drug2 is?significantly different?from drug1 at 0.05 level,which contradict
to the result of anova.

Can you give me some help about that ?
The anova tests the main effect of drug (without any other variable in the 
model) which is non-significant (62 vs. 64 observations).

The summary gives the drug2 effect which pertains to the reference group 
(i.e., result = 1) and this is significant (8 vs. 20 observations).

However, either result is probably not interesting. I guess the 
interaction (and possibly the result main effect) is the quantity of 
interest.
Z
Many thanks.

Best

At?2013-05-13?18:56:01,"Achim?Zeileis"?<Achim.Zeileis at uibk.ac.at>?wrote:
On?Mon,?13?May?2013,?meng?wrote:

?Hi?all:
?I?have?a?question?about?poisson?regression.

?My?data:
?drug?result?count
?1???????1??????????8
?1???????2????????54
?2???????1???????20
?2???????2??????44

?My?model:
?model<-?glm(count?~?drug*result,?family?=?poisson)

?My?result:
?summary(model)

?Coefficients:
??????????????Estimate?Std.?Error?z?value?Pr(>|z|)
?(Intercept)?????2.0794?????0.3536???5.882?4.06e-09?***
?drug2???????????0.9163?????0.4183???2.190???0.0285?*
?result2?????????1.9095?????0.3788???5.041?4.64e-07?***
?drug2:result2??-1.1211?????0.4650??-2.411???0.0159?*

?Calculation?of?coefficients:
?Intercept:?drug=1?and?result=1,the?corresponding?count?is?8.So?log(8)?is?
2.079442?which?is?Intercept.
?My?question:?How?is?drug2?calculated??log(64)-log(62)?or?something?like?
?that(indicates?the?difference?between?drug2?and?drug1)???I've?searched?
?and?tried?many?times?for?all?possible?solutions,but?the?result?is?not?
?0.9163.So,how?is?drug2?calculated??
It's?the?drug2?effect?(on?a?log-scale)?for?result=1,?i.e.,
log(20)?-?log(8).

The?four?coefficients?are:

R>?y?<-?c(8,?20,?54,?44)
R>?c(log(y[1]),?log(y[2])?-?log(y[1]),?log(y[3])?-?log(y[1]),
+????(log(y[4])?-?log(y[3]))?-?(log(y[2])?-?log(y[1])))
[1]??2.0794415??0.9162907??1.9095425?-1.1210851

?Many?thanks!

?Best
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question about poisson regression

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