The Analysis

First, some boxplots.

> motif()
> attach(singers.frame)
> plot(Part, Height)
> title(main="Singers' Heights")

This shows that people who sing the lower parts are generally taller. Of course, part of this is the fact that men sing bass and tenor while women sing alto and soprano. So the interesting comparisons are between bass and tenor, and between alto and soprano.

Note that if part had not been coded as an ordered factor the boxplots would have been in alphabetical order. If it had not been coded as a factor at all, S-PLUS would have made dotplots instead of boxplots.

> t.test(Height[Part=="soprano"],Height[Part=="alto"])

         Standard Two-Sample t-Test

data:  Height[Part == "soprano"] and Height[Part == "alto"]
t = -1.1289, df = 69, p-value = 0.2628
alternative hypothesis: true difference in means is not equal to 0
95 percent confidence interval:
 -1.7591030  0.4876745
sample estimates:
 mean of x mean of y
     64.25  64.88571

> t.test(Height[Part=="tenor"],Height[Part=="bass"])

         Standard Two-Sample t-Test

data:  Height[Part == "tenor"] and Height[Part == "bass"]
t = -2.1297, df = 57, p-value = 0.0375
alternative hypothesis: true difference in means is not equal to 0
95 percent confidence interval:
 -3.04220979 -0.09368765
sample estimates:
 mean of x mean of y
     69.15  70.71795

These t-tests show that there is no significant difference in mean height between sopranos and altos, but that mean height for basses is significantly higher than mean height for tenors.