Normal probability plot examples for Wikipedia (was "qqnorm with histogram?")

Spencer Graves <spencer.graves <at> structuremonitoring.com> writes:

Hi, Jim et al.:

       Thanks for the reply, Jim.

       What are your favorite examples using normal probability plots to 
identify outliers, skewness, kurtosis, mixtures, and the need for 
transformations in plots of raw data and residuals from model fits -- or 
using half-normal plots with estimated parameters?

Not sure I have a *favorite*, but the so-called "Fisher's iris data" 
provide an illustration of an obvious mixture and the value of conditioning.

Try:

 qqnorm(iris$Petal.Length,col=as.numeric(iris$Species))
 qqnorm(residuals(lm(Petal.Length~Species,iris)))

As a bonus, you can reference this pretty page:

 http://en.wikipedia.org/wiki/Iris_%28plant%29

where the contributions of the iris to water purification, art, and
symbolism are noted. 

Maybe a section on the iris in statistics... ;-)


HTH,

Chuck