Year	Number
1971	30.82186
1972	29.87328
1973	29.54177
1974	28.9716
1975	27.68915
1976	25.2317
1977	23.76631
1978	22.3016
1979	21.01156
1980	19.50226
1981	18.31653
1982	17.74149
1983	17.61113
1984	17.81145
1985	17.77242
1986	16.76141
1987	17.05921
1988	17.04171
1989	17.30622
1990	17.36377
1991	17.16923
1992	16.55865
1993	16.23229
1994	15.00398
2011	15.30897
2012	14.84008
2013	15.44236
2014	15.49556
2015	15.5013
2016	15.73516
2017	15.88644