For which x do you think sd(x) is evaluated?
Hint: the help page shows
expr: an expression written as a function of 'x', or alternatively
the name of a function which will be plotted.
and you have written dnorm(x, mean=mean(x), sd=sd(x)) as function of x.
On Thu, 10 Feb 2005, Thomas Hopper wrote:
I am attempting to wrap the histogram function in my own custom function, so
that I can quickly generate some standard plots.
A part of what I want to do is to draw a normal curve over the histogram:
x <- rnorm(1000)
hist(x, freq=F)
curve(dnorm(x), lty=3, add=T)
(for normal use, x would be a vector of empirical values, but the rnorm()
function works for testing)
That works just as you'd expect, but I've found something a bit strange.
If I try the following:
curve(dnorm(x, mean=mean(x), sd=sd(x)), lty=3, add=T)
I get a much flatter and broader curve (which looks like it probably has the
same area as the first curve, though I haven't tested).
However, if I do
z <- sd(x)
curve(dnorm(x, mean=mean(x), sd=z), lty=1, add=T)
I get the curve you'd expect; it draws right over the first curve
(curve(dnorm(x),...), above).
I haven't touched x between the call to curve() containing
dnorm(...,sd=sd(x)) and the call to curve() containing dnorm(...,sd=z), and
tests show that z == sd(x).
I get similar results if I manually type in the standard deviation of x--the
expected curve is drawn--so the broader and flatter curve is only drawn when
I call dnorm with sd=sd(x).
Is there a reason for this, or is there something odd going on with the call
to curve()?
It's working as documented.
--
Brian D. Ripley, [EMAIL PROTECTED]
Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
University of Oxford, Tel: +44 1865 272861 (self)
1 South Parks Road, +44 1865 272866 (PA)
Oxford OX1 3TG, UK Fax: +44 1865 272595
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