In this chapter we have used both Minitab and R to construct normal scores plots. In R
this is done using the function qqnorm, which automatically generates the plot. Before we
show its use, we have three important points to note regarding normal scores plots made
with computer packages. First, different packages handle ties in different ways. The different
approaches result in similar-looking plots, however, so we will not dwell on this point. More
noteworthy is the fact that some packages reverse the axes: they plot the population quantiles
on the vertical axis and the sample quantiles on the horizontal axis. In these cases, the slope
and intercept for such a plot have different meanings than for the plots we have described;
however, the key point remains the same: data from a normal population should result in a
plot where the points follow a straight line.
Our third point is that, instead of using the population quantiles z[(i−0.5)/n] we discussed
earlier, some computer packages use slight variants on this. For example, R uses
z[(i−3/8)/(n+1/4)] for small sample sizes. The reasons for these variants are somewhat technical
and do not concern us here. The differences in the resulting plots are very small.
With these points in mind, we now describe the construction of normal scores plots in R.
Consider some artificial data called mydata. To produce a normal scores plot, you type: