# Interpreting plot of residuals vs. fitted values from Poisson regression

I am trying to fit data with a GLM (poisson regression) in R. When I plotted the residuals vs the fitted values, the plot created multiple (almost linear with a slight concave curve) "lines". What does this mean?

library(faraway)
modl <- glm(doctorco ~ sex + age + agesq + income + levyplus + freepoor +
freerepa + illness + actdays + hscore + chcond1 + chcond2,
family=poisson, data=dvisits)
plot(modl)


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I don't know if you can upload the plot (sometimes newcomers can't), but if not, could you at least add some data & R code to your question so people can evaluate it? –  gung Mar 22 '12 at 15:46
Jocelyn, I've updated your post with information you put in a comment. I also tagged this as homework since you talked about an assignment. –  chl Mar 22 '12 at 16:20
Thanks so much! –  jocelyn Mar 22 '12 at 16:22
From the question, I'm going to assume that you understand the Poisson distribution & Pois reg, and what a plot of residuals vs fitted values tells you (update if that's wrong), thus you are just wondering about the odd appearance of the points in the plot. B/c this is homework, we don't quite answer as our general policy, but provide hints. I notice that you have a lot of covariates, I wonder if you have 1 continuous & many binary covariates. –  gung Mar 22 '12 at 17:45