| Periods | N | Mean | Std. Deviation | Std. Error Mean | ||
| 1961-1976 | 16 | 3.3692E2 | 95.36578 | 23.84145 | ||
| 1977-1993 | 16 | 2.7376E2 | 77.79139 | 19.44785 | ||
| 1994-2009 | 16 | 2.6030E2 | 71.20219 | 17.80055 | ||
| Periods | t | df | Sig. (2-tailed) | Mean Difference | 95% Confidence Interval of the Difference | |
| Lower | Upper | |||||
| 1961-1976 | 14.132 | 15 | 0 | 374.925 | 334.1082 | 414.7418 |
| 1977-1993 | 15.379 | 15 | 0 | 350.7625 | 313.8215 | 387.7035 |
| 1994-2009 | 13.385 | 15 | 0 | 325.3 | 294.8479 | 356.7521 |