| Periods | N | Mean | Std. Deviation | Std. Error Mean | ||
| 1966-1980 | 15 | 1.28E+03 | 0.89083 | 0.23001 | ||
| 1981-1995 | 15 | 1.28E+03 | 0.80487 | 0.20782 | ||
| 1996-2010 | 15 | 1.27E+03 | 1.96535 | 0.50745 | ||
| Periods | t | df | Sig. (2-tailed) | Mean Difference | 95% Confidence Interval of the Difference | |
| Lower | Upper | |||||
| 1966-1980 | 5.55E+03 | 14 | 0 | 1275.74133 | 1275.248 | 1276.2347 |
| 1981-1995 | 6.14E+03 | 14 | 0 | 1276.144 | 1275.6983 | 1276.5897 |
| 1996-2010 | 2.51E+03 | 14 | 0 | 1274.17933 | 1271.091 | 1275.2677 |