Year	Number
1973	15.51463
1974	15.55942
1975	16.31511
1976	17.05477
1977	16.84505
1978	16.41681
1979	16.58828
1980	16.44856
1981	16.06007
1982	16.36318
1983	16.61689
1984	16.5218
1985	16.70899
1986	16.73789
1987	16.73113
1988	16.53817
1989	16.87871
1990	17.00849
1991	15.7991
1992	15.86375
1993	16.51676
1994	16.63862
1995	16.41892
1996	16.09355
1997	16.24066
1998	16.67233
1999	17.00439
2000	16.82987
2001	16.66201
2002	16.74107
2003	16.25119
2004	16.88791
2005	16.80967
2006	17.27311
2007	17.14522
2008	16.971
2009	16.00825
2010	16.35909
2011	16.29746
2012	15.88324
2013	15.46695
2014	15.47788
2015	15.16836
2016	15.20686
2017	15.21963
2018	15.35174