The Minnesota high school graduates of 1938 were classified according to
  four factors, described below.  The minn38 data frame has 168
  rows and 5 columns.
minn38This data frame contains the following columns:
hshigh school rank: "L", "M" and "U" for lower,
      middle and upper third.
phspost high school status: Enrolled in college, ("C"), enrolled in
      non-collegiate school, ("N"), employed full-time, ("E")
      and other, ("O").
folfather's occupational level, (seven levels, "F1", "F2",
      ..., "F7").
sexsex: factor with levels"F" or "M".
ffrequency.