On 18/11/2015 3:49 AM, Luca Cerone wrote:
Dear all,
this is probably a very naive question but I can't understand what
hist() means by density.
A very simple example:
h <- hist(c(1,1,2,3), plot=F)
h$counts
[1] 2 1 0 1
h$density
[1] 1.0 0.5 0.0 0.5
The counts are as I expect, but density is quite puzzling for me.
I would have expected to obtain the probability of that bin (i.e. 0.5,
0.25, 0, 0.25),
but I can't understand how those numbers come out.
The bins are 0.5 wide (see h$breaks). Density has the usual meaning for
continuous distributions: probability per unit. So a density of 1 per
unit over a distance of 0.5 gives a probability of 0.5.
Sometimes sum(h$density) is equal to 1 as I would expect, though.
sum(h$density) would rarely make sense to calculate, any more than the
sum of the normal density function at 4 points would. You want to
integrate a density. The formula for that is
sum(h$density*diff(h$breaks)).
Duncan Murdoch
What am I misunderstanding here?
Thanks a lot for the help!
Cheers,
Luca
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