I would recommend reading the well written page
http://www.stata.com/support/faqs/graphics/histvary.html
and paying special attention to the equal probability version
(eqprhistogram); it has a lot going for it, including its
dislike for zero-height columns.
m.p.
Jann Ben wrote:
Bang! I don't agree. The purpose of a histogram is to make
visible the shape of a density. It is therefore natural to
report the y-axis in terms of a density.
ben
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of
Allan Reese (Cefas)
Sent: Thursday, October 27, 2005 3:03 PM
To: [email protected]
Subject: st: Y axis values for hist ,density
The default "hist x" command in Stata gives a Y axis labelled
a density. I've never given it much attention until I saw
the scale went up to 2 on a plot. Hold on, density functions
sum to 1 over the variable.
Further investigation and discussion with Statacorp
identified that the default tries to make the "area" of the
bars add up to 1. If the number of bars changes, so does
their width and so does the Y labelling. In my example, the
data were discrete, so increasing the number of intervals did
not change the plot except to add more zero-height columns
and hence make each column narrower.
hist x, bin(n) therefore caused different Y
labelling with varying n
hist x, xcale(xrange(0 n) did not affect the labelling,
though the bars got narrower with bigger n
hist x, frac and
hist x, discrete both gave correct labelling, and
the sum of column heights was 1.
Do other users think this is perverse behaviour, especially
as the default? My take is that, when drawing a histogram,
the column width is taken as an arbitrary unit, not directly
related to the x-scale. The implication is that you need to
scale the height only when there are mixed-width columns, but
would not label the Y axis in "freq/absolute-width" units.
Having "densities" that vary and are in such peculiar units
(1/locust in my example!) does not seem helpful.
Shoot me down
Allan
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