Year	Number
1961	3.9186865597713534
1962	4.195441106690514
1963	4.422109174066022
1964	4.872435197280937
1965	5.253398380573005
1966	5.837017077199949
1967	6.068281804957391
1968	6.410957388081627
1969	6.626567514395789
1970	7.540483350213454
1971	7.939469641301711
1972	9.475584645532225
1973	11.077030324154757
1974	12.26902813328589
1975	14.035005396274224