Year	Number
1961	3.645742782632566
1962	3.8326826167884867
1963	3.844842759588713
1964	3.953254392074917
1965	4.123720379666367
1966	4.5134498574439075
1967	4.81559583632448
1968	5.004142227524273
1969	5.265484249986012
1970	5.6251566023552995
1971	5.422108617248503
1972	5.72480493740435
1973	6.409477703942027
1974	6.521508473034832
1975	6.48158794517232
1976	6.46163991427244
1977	6.612966012876179
1978	6.225298930144745
1979	6.3221221723716
1980	6.434323452905718
1981	6.529677510121774
1982	6.576678730641979
1983	6.502685019443244
1984	6.5258612741633115
1985	6.385105134032722
1986	6.370901116068076
1987	6.4319903968789855
1988	5.821022376066263
1989	5.8762187889896955
1990	5.788414157803016
1991	5.760047281323877