Year	Number
1971	18.48588
1972	18.3491
1973	18.42439
1974	18.4786
1975	18.84076
1976	18.83745
1977	18.83428
1978	18.70238
1979	18.53894
1980	18.44734
1981	18.22359
1982	18.06259
1983	17.88316
1984	17.76493
1985	17.80393
1986	17.32441
1987	17.34334
1988	17.33849
1989	17.30374
1990	17.09316
1991	17.42895
1992	17.31672
1993	17.21463
1994	16.82183
1995	16.87372
1996	16.98055
1997	16.87568
1998	16.67223
1999	16.61035
2000	16.41073
2001	16.35693
2002	16.41298
2003	16.5255
2004	16.30431
2005	16.42819
2006	16.44314
2007	16.33676
2008	15.96028
2009	15.75362
2010	15.48802
2011	15.69331
2012	15.45218
2013	15.20144
2014	15.20525
2015	14.91021
2016	14.50623
2017	14.25567
2018	14.52619