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()?
Regards,
Tom
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