[R] Histogram questions

[Thomas Lumley]

On Mon, 5 Feb 2001, M. Edward (Ed) Borasky wrote:

> Questions on histograms:
> 
> 1. When I run "hist" on a data set, the histogram object produced has two
> variables, "$density" and "$intensities". These appear to be the same:
> 
> $intensities
> [1] 1.7619226436 0.0360495682 0.1967705595 0.0000000000 0.0037551633
> [6] 0.0000000000 0.0007510327 0.0007510327
> 
> $density
> [1] 1.7619226436 0.0360495682 0.1967705595 0.0000000000 0.0037551633
> [6] 0.0000000000 0.0007510327 0.0007510327
> 
> Why are they the same? Are both necessary?

It says in help(hist)

   intensities: same as `density'. Deprecated, but retained for
          compatibility.

 
> 2. I want to plot histograms that are normalized -- that is, the sum of the
> intensities or density is 1.0, so the bars represent probabilities. As it's set
> up now, I have to do this calculation manually. Why can't R do this when it
> calculates the values? If I specify "probability=TRUE", I expect probabilities,
> not some arbitrarily scaled value.

A histogram, like a kernel density, estimates the density, not the
probability.  The bar heights would only add to 1 if the bars all had
width 1.  If you want a barplot, use barplot().
 
> 3. What would be even nicer is to have "hist" and "density" *both* scale their
> outputs as probabilities, so they could be overlaid on a plot together. How
> difficult is this to do?

They both *do* scale their outputs as probability densities so they *can*
be overlaid. They couldn't be overlaid if the bar heights added up to 1,
since then the histogram wouldn't be a density estimate.

Eg
 data(faithful)
 hist(faithful$eruptions,prob=TRUE)
 lines(density(faithful$eruptions))


	-thomas

Thomas Lumley			Asst. Professor, Biostatistics
tlumley at u.washington.edu	University of Washington, Seattle

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