Table 7

IH
Years Mean S.D. n nsc t tsc
1944-1960 2.88 1.41 17 17 3.20 3.20
1971-1991 1.62 0.90 21 21
IHD
Years Mean S.D. n nsc t tsc
1944-1960 6.21 3.93 17 17 3.01 2.92
1971-1991 2.75 2.93 21 17.5

Table 7 Contrast of IH and IHD for periods 1944-1960 and 1971-1991, where n is the number of years, nsc is the number of years adjusted for serial correlation, t is the student t test value and tsc is the student t test number adjusted for serial correlation. Both values of (tsc) are significant at the 0.01 level.