qq function

QQ plot

A QQ plot designed with statistics students in mind

qq(x,ax=NA,leg=NA,cex.leg=0.8)

Arguments

• x: A vector of data
• ax: The name you want to call x for the x-axis (if omitted, defaults to what was passed as the first argument). Useful if the variable is a column in a dataframe.
• leg: Optional argument that places a legend in the top left of the plot with the text given by leg
• cex.leg: Optional argument that gives the magnification of the text in the legend

Details

This function gives a "QQ plot" that is more easily interpreted than the standard QQ plot. Instead of plotting quantiles, it plots the observed values of x versus the values expected had x come from a Normal distribution.

The distribution can be considered approximately Normal if the points stay within the upper/lower dashed red lines (with the possible exception at the far left/right) and if there is no overall global curvature.

References

Introduction to Regression and Modeling

Author(s)

Adam Petrie

Examples

#Distribution does not resemble a Normal
   data(TIPS)
   qq(TIPS$Bill,ax="Bill")
   
   #Distribution resembles aNormal
   data(ATTRACTF)
   qq(ATTRACTF$Score,ax="Attractiveness Score")