Year	Number
1970	22.68257
1971	22.7036
1972	22.51388
1973	22.84304
1974	23.08871
1975	23.34594
1976	23.18086
1977	22.89217
1978	22.74618
1979	22.40635
1980	22.07049
1981	21.81105
1982	21.56684
1983	21.32906
1984	21.12589
1985	21.01532
1986	21.00008
1987	20.84762
1988	20.62803
1989	20.35186
1990	20.07931
1991	19.82976
1992	19.94407
1993	20.02015
1994	19.91447
1995	19.91153
1996	20.14071
1997	19.81476
1998	20.55862
1999	19.74555
2000	19.34611
2001	18.90199
2002	18.09755
2003	17.60687
2004	17.49255
2005	17.79701
2006	17.44175
2007	17.31286
2008	17.27924
2009	16.84485
2010	16.13541
2011	15.67077
2012	15.19024
2013	15.05807
2014	14.96433
2015	15.05424
2016	15.27468
2017	15.29526
2018	15.16523