Year	Number
1970	19.77633
1971	19.87323
1972	19.62942
1973	19.45825
1974	19.27239
1975	19.07304
1976	19.01229
1977	18.74976
1978	18.58093
1979	18.33371
1980	18.09389
1981	17.92523
1982	17.73666
1983	17.52554
1984	17.31372
1985	17.00692
1986	16.80732
1987	16.84631
1988	16.67112
1989	16.44071
1990	16.15228
1991	15.76762
1992	15.59485
1993	15.27928
1994	14.98887
1995	15.25228
1996	15.19775
1997	15.1095
1998	15.13064
1999	14.97835
2000	14.58597
2001	14.28152
2002	14.1123
2003	14.12397
2004	14.11408
2005	13.84741
2006	13.76735
2007	13.48999
2008	13.42462
2009	13.28071
2010	13.07971
2011	13.06676
2012	13.28827
2013	13.60514
2014	13.53942
2015	13.40088
2016	13.16466
2017	13.30876
2018	13.29708