Year	Number
1970	15.92385
1971	16.24282
1972	16.45203
1973	16.49009
1974	16.37319
1975	16.47001
1976	16.72747
1977	16.98142
1978	17.15189
1979	16.96355
1980	16.82469
1981	16.70502
1982	16.57014
1983	16.56853
1984	16.6426
1985	16.78841
1986	16.7685
1987	16.69962
1988	16.55048
1989	16.37279
1990	16.18552
1991	16.21533
1992	16.36444
1993	16.50831
1994	16.66528
1995	16.93109
1996	17.09329
1997	17.34921
1998	17.52516
1999	17.54265
2000	17.74911
2001	18.1802
2002	18.17396
2003	18.09076
2004	18.12423
2005	17.87212
2006	17.66707
2007	17.34902
2008	17.07093
2009	16.88514
2010	16.89097
2011	17.02516
2012	17.13973
2013	17.68115
2014	17.62001
2015	17.41341
2016	17.09064
2017	16.94278
2018	16.9982