Year	Number
1970	21.54971
1971	21.60763
1972	21.33537
1973	21.09716
1974	20.96198
1975	20.77314
1976	20.66128
1977	20.40222
1978	20.2585
1979	20.12814
1980	19.92925
1981	19.76466
1982	19.70955
1983	19.6361
1984	19.65762
1985	19.43171
1986	19.29285
1987	19.27981
1988	19.12836
1989	18.84906
1990	18.58735
1991	18.26946
1992	17.93192
1993	17.47102
1994	17.11805
1995	17.33348
1996	17.35645
1997	17.2426
1998	17.15111
1999	16.9551
2000	16.53301
2001	16.20312
2002	16.02089
2003	15.91851
2004	15.80889
2005	15.55318
2006	15.39496
2007	15.05574
2008	14.93745
2009	14.81432
2010	14.71334
2011	14.64212
2012	14.77627
2013	14.9614
2014	14.92689
2015	14.75172
2016	14.69603
2017	14.85681
2018	14.86965