Year	Number
1961	3.011093502377179
1962	3.456402199528672
1963	3.968871595330739
1964	5.405405405405405
1965	6.973180076628352
1966	8.549618320610687
1967	9.339407744874716
1968	10.59001512859304
1969	11.793519216277318
1970	12.987987987987987
1971	14.17910447761194
1972	15.613382899628252
1973	17.77777777777778
1974	19.92619926199262
1975	22.010271460014675
1976	24.944974321349964
1977	27.146001467351432
1978	29.304029304029307
1979	31.478770131771594
1980	32.942898975109806
1981	35.08771929824561
1982	36.91520467836257
1983	38.74269005847953
1984	40.2046783625731
1985	40.816326530612244
1986	41.54518950437318
1987	42.252906976744185
1988	43.11773255813954
1989	43.08417997097243
1990	38.9822718319107
1991	38.93979057591623
1992	43.178519593613935
1993	36.683107274969174
1994	36.570374923171485
1995	36.43600734843846
1996	36.12629022465088
1997	37.02551337896702
1998	38.04347826086957
1999	38.4873949579832
2000	41.03559870550162