Year	Number
1961	2.8169014084507045
1962	2.9166666666666665
1963	3.0136986301369864
1964	3.1081081081081083
1965	3.2
1966	3.289473684210526
1967	3.3766233766233764
1968	3.4615384615384617
1969	3.5443037974683547
1970	3.6249999999999996
1971	3.7037037037037033
1972	3.7804878048780486
1973	3.8554216867469884
1974	3.9285714285714284
1975	4
1976	3.953488372093023
1977	3.310344827586207
1978	2.5
1979	2.4719101123595504
1980	7.044444444444445
1981	7.582417582417582
1982	8.26086956521739
1983	8.817204301075268
1984	9.361702127659575
1985	9.894736842105264
1986	10
1987	10.309278350515463
1988	10.2880658436214
1989	9.876543209876543
1990	4.11522633744856
1991	3.0864197530864197
1992	2.7777777777777777
1993	2.4691358024691357
1994	2.268041237113402
1995	2.0618556701030926
1996	1.8556701030927836
1997	1.6735537190082646