Year	Number
1970	21.08287
1971	21.10044
1972	20.91869
1973	21.35903
1974	21.4895
1975	21.54036
1976	21.23543
1977	20.80219
1978	20.73668
1979	20.25512
1980	20.02474
1981	19.69189
1982	19.25089
1983	18.95387
1984	18.65544
1985	18.48755
1986	18.34671
1987	18.19853
1988	17.95322
1989	17.66076
1990	17.34726
1991	16.83122
1992	16.91839
1993	16.86555
1994	16.68117
1995	16.66462
1996	16.69319
1997	16.52305
1998	16.16579
1999	15.53303
2000	15.24285
2001	14.81335
2002	14.92757
2003	13.94648
2004	13.87854
2005	14.04909
2006	13.76404
2007	13.59235
2008	13.36385
2009	13.22727
2010	13.14935
2011	13.08802
2012	13.00115
2013	12.69065
2014	12.64786
2015	12.88179
2016	13.01309
2017	12.80551
2018	12.77154