In response to a question from Kin Cheung, Albyn Jones writes:
> a histogram, or in general a density does not have to be bounded
> by 1, the total area has to be 1. Consider for example the normal
> with small standard deviation:
>
> > dnorm(0,0,.001)
> [1] 398.9423
All very true, but I think the argument name ``probability'' in
hist() is misleading. If I set ``probability = T'' I'd expect to get
probabilities, numbers in the range 0 to 1. In other words, what the
elementary stats texts call a ``relative frequency'' histogram.
(Admittedly the documentation for hist() does indeed make it clear
--- if one reads it --- that one is getting a p.d.f. and not relative
frequencies.)
If the histogram is to be thought of as a probability ***density***
function, the argument should ``say so'' --- i.e. the argument should
be ``probability.density = T''. Or, perhaps to shorten things down,
``pdf = T''.
One does, from time to time, want ``relative frequency'' histograms;
it would be nice if hist() provided a facility for this. As things
stand, one has to modify the code of hist() locally, or fiddle about
with barplot(). Neither is difficult to do, but it's a mild pain
in the pohutukawa.
Rolf Turner
rolf@math.unb.ca
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