Year	Number
1961	0.22727272727272727
1962	0.2181691247054717
1963	0.25793139025019346
1964	0.4298856504169891
1965	0.6669445602334306
1966	1.0770505385252693
1967	1.4814814814814816
1968	1.7959183673469388
1969	1.9952114924181963
1970	2.324820430965682
1971	2.4745497259201255
1972	2.5815384615384613