Year	Number
1970	22.02089
1971	20.3819
1972	20.36483
1973	20.29968
1974	20.10415
1975	21.44538
1976	21.38728
1977	20.74596
1978	20.87246
1979	23.02364
1980	22.71411
1981	23.10281
1982	23.03382
1983	23.64302
1984	22.66238
1985	23.01972
1986	24.21917
1987	23.68102
1988	23.32104
1989	22.89541
1991	20.62345
1992	21.5935
1993	21.20383
1994	20.71061
1996	19.33663
1997	19.07165
2013	17.10287
2014	16.99821
2015	16.78552
2016	16.59563
2017	16.47539