Dominique - 8 months ago 33

R Question

I ran a model using

`glmer`

`model <- glmer(Number~Year*Treatment(1|Year/Treatment), data=data,family=poisson)`

This site recommends using

`glmmPQL`

`library(MASS)`

library(nlme)

model1<-glmmPQL(Number~Year*Treatment,~1|Year/Treatment,

family=gaussian(link = "log"),

data=data,start=coef(lm(Log~Year*Treatment)),

na.action = na.pass,verbose=FALSE)

summary(model1)

plot(model1)

Now do you transform the data in the Excel document or in the R code (

`Number1 <- log(Number)`

`link="log"`

If you have data with zeros, is it acceptable to add 1 to all observations to make it more than zero in order to log transform it:

`Number1<-log(Number+1)`

Is

`fit<-anova(model,model1,test="Chisq")`

Many thanks for any advice!

Answer

**tl;dr** your diagnostic plots look OK to me, you can probably proceed to interpret your results.

This formula:

`Number~Year*Treatment+(1|Year/Treatment)`

might not be quite right (besides the missing `+`

between the terms above ...) In general you shouldn't include the same term in both the random and the fixed effects (although there is one exception - *if* `Year`

has more than a few values and there are multiple observations per year you can include it as a continuous covariate in the fixed effects and a grouping factor in the random effects - so this *might* be correct).

- I'm not crazy about the linked introduction; at a quick skim there's nothing
*horribly*wrong with it, but there seem to b e a lot of minor inaccuracies and confusions. "Use`glmmPQL`

if your data aren't Normal" is really shorthand for "you might want to use a GLMM if your data aren't Normal". Your`glmer`

model should be fine. - interpreting diagnostic plots is a bit of an art, but the degree of deviation that you show above doesn't look like a problem.
- since you don't need to log-transform your data, you don't need to get into the slightly messy issue of how to log-transform data containing zeros. In general
`log(1+x)`

transformations for*count data*are reasonable - but, again, unnecessary here. `anova()`

in this context does a likelihood ratio test, which is a reasonable way to compare models.