Year	Number
1971	18.25461
1972	17.71465
1973	17.54871
1974	17.29659
1975	17.05048
1976	16.26652
1977	15.24625
1978	15.03048
1979	14.70217
1980	14.72768
1981	14.51248
1982	14.00031
1983	13.79219
1984	13.36774
1985	13.07814
1986	12.48597
1987	12.15722
1988	12.01703
1989	11.76216
1990	11.21641
1991	10.55668
1992	9.23498
1993	9.59869
1994	9.6582
1995	10.23652
1996	10.64186
1997	10.57744
1998	10.68457
1999	11.3243
2000	11.27836
2001	11.43006
2002	11.41059
2003	11.76673
2004	11.49663
2005	11.48245
2006	11.45731
2007	11.44767
2008	11.43057
2009	11.80169
2010	12.02208
2011	12.29878
2012	12.49293
2013	12.69734
2014	12.82018
2016	12.30445
2017	12.23179