sum(.) on a factor column returns incorrect result

Question

I am in a strange fix here. I am using data.table for a very routine task, but there is something that I am not able to explain. I have figured out a way around

Answer 1

The correct way to convert from factor to numeric or integer is to go through character. This is because internally, a factor is an integer index (that refers to a levels vector). When you tell R to convert it to numeric it will simply convert the underlying index, not try to convert the level label.

Short answer: do Cases:=as.numeric(as.character(Cases)).

Edit: Alternatively the ?factor help page suggests as.numeric(levels(Cases))[Cases] as more efficient. h/t @Gsee in the comments.