[R] Quasipoisson with geeglm

[Søren Højsgaard]

Dear Ivy,

In gee there is no quasipossion, because gee is in a way already quasi.

With GEE we do not fit a poisson glm, but use in the construction of the sandwich covariance matrix the variance function of the poisson family. In Gee always an 'overdispersion' is estimated.

Regards
Søren

________________________________________
Fra: r-help-bounces at r-project.org [r-help-bounces at r-project.org] På vegne af JANSEN, Ivy [Ivy.JANSEN at inbo.be]
Sendt: 7. april 2011 13:32
Til: r-help at r-project.org
Emne: [R] Quasipoisson with geeglm

Dear all,

I am trying to use the GEE methodology to fit a trend for the number of butterflies observed at several sites. In total, there are 66 sites, and 19 years for which observations might be available. However, only 326 observations are available (instead of 1254). For the time being, I ignore the large number of missing values, and the fact that GEE is only valid under MCAR. When I run the following code

geeglm(SumOfButterflies ~ RES_YEAR, family = poisson, data = ManijurtNoNA, id = RES_ROTE_ID, corstr = "ar1")

I obtain "normal" output. Not surprisingly, overdispersion is present (Estimated Scale Parameters:  [1] 185.8571), so changing to quasipoisson is needed. However, the code below

geeglm(SumOfButterflies ~ RES_YEAR, family = quasipoisson, data = ManijurtNoNA, id = RES_ROTE_ID, corstr = "ar1")

produces the following error

Error in geese.fit(xx, yy, id, offset, soffset, w, waves = waves, zsca,  :   variance invalid.

Other correlation structures encounter the same problem. I also tried adding "waves = RES_YEAR" (although I am not sure how waves should be used correctly), but it does not change anything.

Any suggestions what might be wrong?

Regards,
Ivy



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