Periods	N	Mean	Std. Deviation	Std. Error Mean
1961-1976	16	12.0267	0.92421	0.23105
1977-1993	16	12.4541	0.70933	0.17733
1994-2009	16	13.3933	0.69768	0.17442
Periods	t	df	Sig. (2-tailed)	Mean Difference	95% Confidence Interval of the Difference
Periods	t	df	Sig. (2-tailed)	Mean Difference	Lower	Upper
1961-1976	52.052	15	0	12.02669	11.5342	12.5192
1977-1993	70.23	15	0	12.45407	12.0761	12.832
1994-2009	76.788	15	0	13.39329	13.0215	13.7651