Year	Number
1973	14.96229
1974	15.1962
1975	15.30835
1976	16.94664
1977	19.13623
1978	17.78223
1979	17.68876
1980	18.14563
1981	18.69921
1982	18.10688
1983	18.19534
1984	17.97135
1985	17.99091
1986	18.21044
1987	18.46991
1988	18.7168
1989	18.59718
1990	18.1942
1991	17.98862
1992	18.71686
1993	19.74657
1994	19.84917
1995	20.06955
1996	20.33604
1997	20.19541
1998	20.51804
1999	20.00526
2000	19.9435
2001	19.53943
2002	19.12194
2003	18.2247
2004	18.22994
2005	17.559
2006	18.80959
2007	18.57224
2008	18.49587
2009	18.42962
2010	18.67534
2011	18.38212
2012	18.28684
2013	18.85669
2014	18.3194
2015	17.70526
2016	17.30015
2017	16.82072
2018	16.52367