Year	Number
1970	20.95226
1971	21.06412
1972	20.68939
1973	20.73073
1974	20.67932
1975	20.43739
1976	20.23342
1977	19.78016
1978	19.38574
1979	19.01197
1980	18.61357
1981	18.89266
1982	18.72027
1983	18.49983
1984	18.32953
1985	18.02709
1986	17.82325
1987	17.6684
1988	17.50972
1989	17.8326
1990	17.72497
1991	17.57421
1992	17.60595
1993	17.69038
1994	17.69494
1995	17.8478
1996	18.15333
1997	18.03269
1998	18.78214
1999	18.17288
2000	17.83537
2001	17.5316
2002	16.69896
2003	16.19278
2004	15.65831
2005	15.84375
2006	15.38681
2007	15.18063
2008	15.19311
2009	14.85068
2010	14.27053
2011	13.89732
2012	13.4714
2013	13.49878
2014	13.43726
2015	13.58181
2016	13.80744
2017	13.8489
2018	13.67607