Year	Number
1970	20.30554
1971	21.23448
1972	21.94234
1973	20.90399
1974	20.10994
1975	20.39019
1976	21.20685
1977	21.1884
1978	21.00015
1979	20.39096
1980	19.23391
1981	18.41479
1982	17.55115
1983	17.31104
1984	17.30607
1985	17.60679
1986	17.45393
1987	17.29038
1988	16.79728
1989	15.97753
1990	15.05413
1991	15.04778
1992	15.28672
1993	15.47085
1994	15.56814
1995	16.1246
1996	16.56255
1997	17.05356
1998	17.28103
1999	17.26358
2000	17.89193
2001	19.12002
2002	19.0019
2003	18.8937
2004	18.76105
2005	18.26885
2006	17.66667
2007	16.85046
2008	16.35692
2009	16.2509
2010	15.92361
2011	16.16995
2012	16.00089
2013	16.30567
2014	15.83737
2015	15.34908
2016	14.90642
2017	15.01544
2018	14.98305