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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