[R] Question concerning side effects of treating invalid factor levels

[tibor@kiss m@iii@g oii rub@de]

Dear List members,

I have tried now for several times to find out about a side effect of treating invalid factor levels, but did not find an answer. Various answers on stackexchange etc. produce the stuff that irritates me without even mentioning it. 
So I am asking the list (apologies if this has been treated in the past).

If you add an invalid factor level to a column in a data frame, this has the side effect of turning a numerical column into a column with character strings. Here is a simple example:

> df <- data.frame(
	P = factor(c("mittels", "mit", "mittels", "ueber", "mit", "mit")),
	ANSWER = factor(c(rep("PP>OBJ", 4), rep("OBJ>PP", 2))), 
	RT = round(runif(6, 7000, 16000), 0))

> str(df)
'data.frame':	6 obs. of  3 variables:
$ P     : Factor w/ 3 levels "mit","mittels",..: 2 1 2 3 1 1
$ ANSWER: Factor w/ 2 levels "OBJ>PP","PP>OBJ": 2 2 2 2 1 1
$ RT    : num  11157 13719 14388 14527 14686 ..

> df <- rbind(df, c("in", "V>N", round(runif(1, 7000, 16000), 0)))

> str(df)
'data.frame':	7 obs. of  3 variables:
$ P     : Factor w/ 3 levels "mit","mittels",..: 2 1 2 3 1 1 NA
$ ANSWER: Factor w/ 2 levels "OBJ>PP","PP>OBJ": 2 2 2 2 1 1 NA
$ RT    : chr  "11478" "15819" "8305" "8852" …

You see that RT has changed from _num_ to _chr_ as a side effect of adding the invalid factor level as NA. I would appreciate understanding what the purpose of the type coercion is.

Thanks in advance


Tibor