Year	Number
1970	18.35076
1971	18.907
1972	18.21737
1973	20.02735
1974	20.02224
1975	19.27626
1976	18.58139
1977	19.10178
1978	18.60125
1979	19.2838
1980	20.51473
1981	19.52927
1982	19.36475
1983	18.10532
1984	16.98813
1985	18.21056
1986	17.68734
1987	17.42243
1988	17.29403
1989	18.62241
1991	18.61904
1992	18.69583
1993	18.48356
1994	17.08414
1995	17.33276
1996	16.73171
1997	18.95695
2014	23.51587
2016	15.93942
2017	17.11346
2018	20.86254