Somehow it seems that tapply converts factors given as the _first_
argument to a numerical representation without the old factor levels.
> unlist(lapply(Machines, data.class))
Worker Machine score
"ordered" "factor" "numeric"
> Machines[ 1:10, ]
Grouped Data: score ~ 1 | Worker
Worker Machine score
1 1 A 52.0
2 1 A 52.8
3 1 A 53.1
4 2 A 51.8
5 2 A 52.8
6 2 A 53.1
7 3 A 60.0
8 3 A 60.2
9 3 A 58.4
10 4 A 51.1> tapply( Machines$Machine, Machines$Worker, function(x) x )
$6
[1] 1 1 1 2 2 2 3 3 3
$2
[1] 1 1 1 2 2 2 3 3 3
$4
[1] 1 1 1 2 2 2 3 3 3
$1
[1] 1 1 1 2 2 2 3 3 3
$3
[1] 1 1 1 2 2 2 3 3 3
$5
[1] 1 1 1 2 2 2 3 3 3
> tapply( Machines$Machine, Machines$Worker, data.class )
6 2 4 1 3 5
"factor" "factor" "factor" "factor"
"factor" "factor" > tapply( Machines$Machine, Machines$Worker, levels )
$6
character(0)
$2
character(0)
$4
character(0)
$1
character(0)
$3
character(0)
$5
character(0)> levels( Machines$Machine )
[1] "A" "B" "C"
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