Year	Number
1970	20.5079
1971	21.48689
1972	22.22107
1973	21.08598
1974	20.24353
1975	20.52646
1976	21.35757
1977	21.31147
1978	21.10667
1979	20.48374
1980	19.29456
1981	18.44883
1982	17.55406
1983	17.30436
1984	17.29919
1985	17.6118
1986	17.45312
1987	17.28447
1988	16.77792
1989	15.93926
1990	14.99849
1991	14.99589
1992	15.24172
1993	15.43044
1994	15.52937
1995	16.09681
1996	16.544
1997	17.04509
1998	17.27727
1999	17.25964
2000	17.89966
2001	19.15103
2002	19.02937
2003	18.91804
2004	18.7824
2005	18.28174
2006	17.6697
2007	16.84127
2008	16.34081
2009	16.22447
2010	15.90194
2011	16.15151
2012	15.98017
2013	16.28906
2014	15.81443
2015	15.32017
2016	14.87222
2017	14.98327
2018	14.95966