My data frame looks like:
head(bush_status)
distance status count
0 endemic 844
1 exotic 8
5 native 3
10 endemic 5
15 endemic 4
20 endemic 3
The count data is non-normally distributed. I'm trying to fit a generalized additive model to my data in two ways so i can use anova to see if the p-value supports m2.
m1 <- gam(count ~ s(distance) + status, data=bush_status, family="nb")
m2 <- gam(count ~ s(distance, by=status) + status, data=bush_status, family="nb")
m1 works fine, but m2 sends the error message:
"Error in smoothCon(split$smooth.spec[[i]], data, knots, absorb.cons,
scale.penalty = scale.penalty, :
Can't find by variable"
This is pretty beyond me so if anyone could offer any advice that would be much appreciated!
From your comments it became clear that you passed a character variable to by in the smoother. You must pass a factor variable there. This has been a frequent gotcha for me too and I consider it a design flaw (because base R regression functions deal with character variables just fine).