Year	Number
1970	13.91574
1971	14.16065
1972	14.44469
1973	14.83846
1974	15.08065
1975	15.38792
1976	15.48948
1977	15.54109
1978	15.86864
1979	15.5959
1980	15.81014
1981	15.94603
1982	16.07542
1983	16.21331
1984	16.28136
1985	16.42641
1986	16.34685
1987	16.29846
1988	16.37773
1989	16.49702
1990	16.59482
1991	16.76116
1992	16.92607
1993	17.07407
1994	17.1771
1995	17.37002
1996	17.45597
1997	17.5698
1998	17.7113
1999	17.74227
2000	18.01969
2001	18.76218
2002	18.81616
2003	17.6023
2004	17.44389
2005	17.14483
2006	17.91904
2007	18.49472
2008	17.76674
2009	17.74325
2010	17.50221
2011	17.27736
2012	17.08228
2013	17.46553
2014	16.69484
2015	16.56983
2016	16.5865
2017	16.67372
2018	16.65098