How Does Minitab Choose Class Intervals for a Histogram?

Here is the email I received from Minitab Technical Support:

Date: Fri, 26 Sep 1997 15:07:50 -0400
To: jcryer@stat.uiowa.edu
From: Tech Support 
Subject: number of bars in MINITAB histogram

Jonathan,

I finally found an answer for you.  Here's the algorithm.

There are upper and lower bounds on the number of bars.

Lower bound = Round( (16.0*N)**(1.0/3.0) + 0.5 )
Upper bound = Lower bound + Round(0.5*N)

After you find the bounds, MINITAB will always try to get as close to the
lower bound as it can.

Then we have a "nice numbers" algorithm that finds interval midpoints,
given the constraints on the number of intervals.

But there is special code for date/time data and for highly granular data
(e.g., all 1's and 2's).

Find the largest integer p such that each data value can be written (within
fuzz) as an integer times 10**p.

Let BinWidth = 10**p.

Let BinCount =  1 + Round( ( range of data ) / BinWidth )

If BinCount  is <= 10, then let the bin midpoints run from the data min to
the data max in increments of BinWidth.

Otherwise, use the "nice numbers" algorithm.

Hope this helps.

Andy Haines