[R] unexpected breaks in hist
Paul Murrell
paul at stat.auckland.ac.nz
Mon May 7 04:43:21 CEST 2001
Hi
> I've been using hist to plot histograms of some data, but I get variable
> numbers of bins. I understood from reading the help file that breaks will
> set the number of bins. Please correct me if I'm wrong.
> I'm plotting measures from subsets of a larger data set. Depending on the
> data subset the number of bins varies despite having set breaks to 5 in
> all cases.
The crucial part of the help is the word "approximate" ...
breaks: either a single number giving the approximate number of cells
for the histogram or a vector giving the breakpoints between
histogram cells.
... R takes your suggestion for the number of bins and modifies it if
necessary to produce a "nice" number of bins.
You can have exact control over the number of bins if you give the bin
breaks. For example, compare ...
par(mfrow=c(2,2))
for (i in 1:4)
hist(rnorm(50), breaks=4) # sometimes 6 bins, sometimes 5 bins, ...
... with ...
par(mfrow=c(2,2))
for (i in 1:4)
hist(rnorm(50), breaks=c(-4,-2,0,2,4)) # always 4 bins
Hope that helps
Paul
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