| 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.