Year	Number
1961	3.5199896505431587
1962	3.727751985280799
1963	3.7075007938525095
1964	3.8206504377934474
1965	4.0022476786121075
1966	4.5955862724432235
1967	4.9002581008327715
1968	5.018404737250551
1969	5.340322268798386
1970	6.0973441104282164
1971	6.1000848372868655
1972	6.244096922404512
1973	6.04166033453897
1974	6.5373486798714975
1975	6.68007293007293
1976	6.646576277695401
1977	7.193008292174543
1978	7.479289940828402
1979	7.654931450540416
1980	7.9889904372066765
1981	8.791354374946334
1982	9.198639999032027
1983	10.06196905157269
1984	9.817563817980021
1985	9.634672463064515
1986	9.483012830567494
1987	9.485932942320359