I have constructed a GAM using the package mgcv to test whether the lengths of 
an emerging insect (Length) varies with day of the year (DOY) and between two 
sites (SiteCode). The data are collected at irregular time steps ranging from 2 
days to 20 days between samples. The GAM takes the form

M3 <- gam(Length ~s(DOY, by = SiteCode) + SiteCode)

As the data are a time series I would like to test for temporal 
autocorrelation. I have read that it is not possible to use the autocorrelation 
function (ACF) due to the irregular spacing and that producing a variogram in 
relation to DOY would be an option.

Is this a correct method to test for temporal autocorrelation?

And could someone suggest the code to produce the variogram as I'm getting an 
error related to the 'distance' argument

Best wishes
Tom



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