Year	Number
1973	15.72733
1974	15.98386
1975	16.05482
1976	17.15788
1977	18.83225
1978	17.66752
1979	17.66188
1980	18.11935
1981	18.83965
1982	18.31544
1983	18.20891
1984	18.00949
1985	17.9298
1986	18.06629
1987	18.16607
1988	18.33347
1989	18.45498
1990	17.97104
1991	17.78721
1992	18.31946
1993	19.20895
1994	19.3775
1995	19.56957
1996	19.6871
1997	19.44604
1998	19.43814
1999	18.4883
2000	17.23932
2001	17.60867
2002	17.19324
2003	16.66008
2004	16.45529
2005	15.82868
2006	16.71801
2007	16.68909
2008	16.68393
2009	17.17366
2010	17.71043
2011	17.65156
2012	17.86461
2013	18.42064
2014	17.90373
2015	17.60876
2016	17.37288
2017	16.95639
2018	16.72069