Year	Number
1971	19.34683
1972	18.32957
1973	17.59063
1974	18.81847
1975	20.34956
1976	22.87447
1977	22.76105
1978	21.98055
1979	21.96051
1981	21.55264
1982	21.44506
1984	23.43608
1986	22.84268
1987	21.69439
1988	21.40405
1989	21.9192
1991	21.95705
1992	21.13453
1998	22.24673
2012	27.40981
2016	34.2964