Year	Number
1971	35.81879
1972	40.26852
1973	38.26171
1974	37.43045
1975	34.17873
1976	33.00662
1977	32.87126
1978	31.92045
1979	29.30962
1980	26.85446
1981	26.59159
1982	28.37751
1983	26.85012
1984	26.25962
1985	25.81611
1986	20.92365
1987	23.94245
1988	20.93477
1989	23.76803
1990	23.52256
1991	23.20538
1992	23.36738
1993	25.93452
1994	23.11974
1995	22.44712
1996	22.20857
1997	22.40174
1998	20.74866
1999	22.20961
2000	21.88032
2002	21.14914
2003	20.27444
2004	20.36357
2005	20.35219
2006	20.35182
2007	19.28755
2008	20.6876
2010	18.42929
2011	16.94387
2012	16.93804
2013	16.3791
2014	17.7286
2015	17.14
2016	16.95452
2017	17.87505
2018	17.55842