[R] How hist() decides breaks?

From: Ted Harding <Ted.Harding_at_manchester.ac.uk>
Date: Mon, 19 May 2008 10:31:24 +0100 (BST)

Hi Folks,
I'd like to know how hist() decides how many cells to use when it ignores my "suggestion" to use say 'hist(...,breaks=50)'.

More specifically, I have the results of 10000 simulations, each returning an 8-vector, therefore 8 variables each with 10000 values. Some of these 8 have somewhat skew distributions. Say one of these 8 variables is X.

I ask for H <- hist(X,breaks=50), and get a histogram which usually has a different number of cells than what I intended.

For instance, for one of these simulations, the 8 different values of length(H$breaks) are:

  70, 44, 38, 68, 50, 40, 46, 45

?hist tells me

  breaks: one of:

    In the last three cases the number is a suggestion only.

  The default for 'breaks' is '"Sturges"': see 'nclass.Sturges'.

If I look at the code for nclass.Sturges() I see

  function (x) ceiling(log2(length(x)) + 1)

and, for length(X) = 10000, this gives 15. This is not related to any of the numbers of breaks I actually got, in any way obvious to me.

Question 1: hist() has apparently ignored my "suggestion" of   "break=50". Why? What is the criterion for ignoring?

Question 2: Presumably, if it ignores the "suggestion", it   does something else, of its choice. I would then, perhaps,   expect it to fall back to its default, which is (allegedly)   Sturges. But the result from nclass.Sturges looks different   from what it actually did. So what did it actually do, and   how did it decide on this?

With thanks,

E-Mail: (Ted Harding) <Ted.Harding_at_manchester.ac.uk> Fax-to-email: +44 (0)870 094 0861
Date: 19-May-08                                       Time: 10:31:20
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