There may be a bug in the anova.glm function.
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> counts <- c(18,17,15,20,10,20,25,13,12)
> outcome <- gl(3,1,9)
> treatment <- gl(3,3)
> glm.D93 <- glm(counts ~ outcome + treatment, family=poisson())
> anova(glm.D93,test="Chisq")
Analysis of Deviance Table
Model: poisson, link: log
Response: counts
Terms added sequentially (first to last)
Df Deviance Resid. Df Resid. Dev P(>|Chi|)
NULL 8 10.5814
outcome 2 5.4523 6 5.1291 0.0655
treatment 2 0.0000 4 5.1291 1.0000
> anova(glm.D93,test="F")
Analysis of Deviance Table
Model: poisson, link: log
Response: counts
Terms added sequentially (first to last)
Df Deviance Resid. Df Resid. Dev F Pr(>F)
NULL 8 10.5814
outcome 2 5.4523 6 5.1291 2.7262 0.06547 .
treatment 2 0.0000 4 5.1291 0.0000 1.00000
---
Signif. codes: 0 `***' 0.001 `**' 0.01 `*' 0.05 `.' 0.1 ` ' 1
----------------------------------------------------------------------
The F test should use the estimated dispersion parameter, so should be
different from the Chisq test. The following is what I got from Splus:
deathstar[31] Splus
S-PLUS : Copyright (c) 1988, 2000 MathSoft, Inc.
S : Copyright Lucent Technologies, Inc.
Version 6.0 Release 1 for Linux 2.2.12 : 2000
Working data will be in /data/home/faculty/yuedong/MySwork
> counts <- c(18,17,15,20,10,20,25,13,12)
> outcome <- factor(rep(1:3,3))
> treatment <- factor(rep(1:3,c(3,3,3)))
> glm.D93 <- glm(counts ~ outcome + treatment, family=poisson())
> anova(glm.D93, test="Chisq")
Analysis of Deviance Table
Poisson model
Response: counts
Terms added sequentially (first to last)
Df Deviance Resid. Df Resid. Dev Pr(Chi)
NULL 8 10.58145
outcome 2 5.452305 6 5.12914 0.0654707
treatment 2 0.000000 4 5.12914 1.0000000
> anova(glm.D93, test="F")
Analysis of Deviance Table
Poisson model
Response: counts
Terms added sequentially (first to last)
Df Deviance Resid. Df Resid. Dev F Value Pr(F)
NULL 8 10.58145
outcome 2 5.452305 6 5.12914 2.108359 0.2369863
treatment 2 0.000000 4 5.12914 0.000000 1.0000000
--------------------------------------------------------------------
Yuedong Wang Phone: (805) 893-4870
Dept of Statistics & Applied Probability Fax: (805) 893-2334
Univ of California [EMAIL PROTECTED]
Santa Barbara, CA 93106 http://www.pstat.ucsb.edu/faculty/yuedong
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