Categories and level semantics

The idea

A factor stores integer codes together with category labels called levels. The codes describe positions in the level set, not numeric measurements. Converting a factor directly with as.numeric therefore returns codes. If numeric-looking labels represent quantities, convert their character labels deliberately and check whether parsing introduced missing values.

Define levels when a domain has a meaningful order or a fixed set of accepted categories. An ordered factor can express low before medium before high without alphabetical sorting. Values outside the supplied levels become NA, which should trigger validation if unknown categories are forbidden. Unused levels may remain after subsetting, so decide whether reports should retain empty categories or drop them.

priority <- factor(c("high","low"), levels=c("low","medium","high"), ordered=TRUE)
cat(as.integer(priority), sep=",")
cat("\n", priority[1] > priority[2], "\n", sep="")

3,1
TRUE

The labels map to explicitly ordered codes three and one, making high greater than low.