Normal Probability Plots

Preliminary Notes

You create a normal probability plot with qqnorm.

qqnorm(data)

There are two strategies for making normal probability plots: You can put the actual values on the X axis and the expected values on the Y axis, or vice versa. By default, the expected values are the x values. You tell R to put the data values along the x axis with datax=T.

qqnorm(data, datax=T)

We may also want to add a label, which we do with the ylab parameter.

qqnorm(data, datax=T, ylab="LABEL")

Finally, it is helpful to put a normal line through the data to help see how much the data deviate. We use qqline for this purpose. (If you've specified datax=T for qqnorm, you should also specify it for qqline.)

qqnorm(data, datax=T, ylab="LABEL")
qqline(data, datax=T)