IH | ||||||

Years | Mean | S.D. | n | n_{sc} | t | t_{sc} |

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 | n_{sc} | t | t_{sc} |

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,
n_{sc} is the number of years adjusted for serial
correlation, t is the student t test value and t_{sc} is
the student t test number adjusted for serial correlation.
Both values of (t_{sc}) are significant at the 0.01
level.