[R] Quasipoisson with geeglm

From: JANSEN, Ivy <Ivy.JANSEN_at_inbo.be>
Date: Thu, 07 Apr 2011 11:32:22 +0000


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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