Year	Number
1971	25.50733
1972	25.09853
1973	24.159
1974	18.95751
1976	20.99507
1977	20.18509
1980	20.68069
1981	21.4974
1982	20.89861
1983	20.70324
1984	24.4335
1985	23.25082
1986	23.62451
1987	23.88034
1988	23.25954
1989	21.78292
1990	23.12055
1991	22.96142
1992	23.53104
1993	22.83655
1994	22.39772
1995	21.97555
1996	21.80037
1997	21.11881
1998	20.85216
1999	19.62563
2000	19.14022
2001	18.99717
2002	19.06585
2003	18.17192
2004	18.78114
2005	18.40547
2006	17.95282
2007	16.85152
2008	17.52094
2010	14.29348
2011	12.61131
2012	11.45141
2013	13.89999
2014	13.38202
2015	13.49445
2016	13.02685
2017	12.93931