Year	Number
1971	19.77837
1972	19.75679
1973	19.89941
1974	20.02049
1975	20.19733
1976	20.11737
1977	20.29065
1978	20.21784
1979	20.40689
1980	20.95409
1981	21.18117
1982	20.98428
1983	21.32059
1984	21.26379
1985	21.72426
1986	20.93509
1987	20.39443
1988	19.63883
1989	19.3313
1990	19.22615
1991	19.37988
1992	19.56908
1993	19.64413
1994	19.94479
1995	20.75747
1996	21.4441
1997	22.05657
1998	22.80498
1999	23.39018
2000	23.65135
2001	23.88236
2002	23.83942
2003	23.50757
2004	23.77725
2005	23.06853
2006	22.94072
2007	22.7184
2008	22.62204
2009	22.64745
2010	22.79844
2011	21.86446
2012	22.99397
2013	22.1521
2014	21.68999
2015	21.40375
2016	21.72611
2017	21.27684
2018	21.68306