[Rd] Error message when calling t.test() and aov() with a factor variables

[Kurt Hornik]

>>>>> Viechtbauer, Wolfgang (NP) writes:

> Hi all,
> Just noticed that the error that arises when calling t.test() with factors could be a bit clearer:

>> t.test(factor(c(3,1,2,4,3,5,4,5)), factor(c(2,1,2,3,4,5)))
> Error in var(x) : Calling var(x) on a factor x is defunct.
>   Use something like 'all(duplicated(x)[-1L])' to test for a constant vector.
> In addition: Warning message:
> In mean.default(x) :
>   argument is not numeric or logical: returning NA

> Obviously, using factors as input is nonsense, but this might happen on accident and then the error message could be a bit more on point. Similar for aov():

>> aov(factor(c(3,1,2,4,3,5,4,5)) ~ factor(c(2,1,2,2,2,1,2,1)))
> Call:
>    aov(formula = factor(c(3, 1, 2, 4, 3, 5, 4, 5)) ~ factor(c(2,
>     1, 2, 2, 2, 1, 2, 1)))
> Error in levels(x)[x] :
>   only 0's may be mixed with negative subscripts
> In addition: Warning messages:
> 1: In model.response(mf, "numeric") :
>   using type = "numeric" with a factor response will be ignored
> 2: In Ops.factor(y, z$residuals) : '-' not meaningful for factors

> Not a big deal and trying to catch all of the silly things users may
> do is of course impossible, but for this one adding a check that the
> (response) variable is actually numeric could be useful.

Indeed.

As always, the question is whether we want to give an error unless
is.numeric, or ensure via as.numeric?

Best
-k

> Best,
> Wolfgang

> --
> Wolfgang Viechtbauer, PhD, Statistician | Department of Psychiatry and
> Neuropsychology | Maastricht University | PO Box 616 (VIJV1) | 6200 MD
> Maastricht, The Netherlands | +31(43)3884170 | https://www.wvbauer.com

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