Year	Number
1961	2.9038461538461537
1962	3.7307692307692304
1963	4.134615384615384
1964	4.749034749034749
1965	7.876447876447877
1966	8.687258687258687
1967	9.922779922779924
1968	10.55984555984556
1969	11.599229287090559
1970	11.884615384615385
1971	12.26923076923077
1972	12.346153846153845
1973	12.32824427480916
1974	12.300380228136882