Year	Number
1970	14.10875
1971	14.30238
1972	14.53313
1973	14.93004
1974	15.14398
1975	15.45277
1976	15.5966
1977	15.60122
1978	15.79981
1979	15.57733
1980	15.79836
1981	15.90997
1982	15.98627
1983	16.10868
1984	16.17532
1985	16.2505
1986	16.20986
1987	16.16078
1988	16.20955
1989	16.29667
1990	16.36833
1991	16.55218
1992	16.70448
1993	16.82927
1994	16.94222
1995	17.20816
1996	17.27193
1997	17.40244
1998	17.58367
1999	17.60807
2000	17.88522
2001	18.56674
2002	18.6151
2003	17.43775
2004	17.26327
2005	16.97222
2006	17.68437
2007	18.18853
2008	17.50713
2009	17.48758
2010	17.25248
2011	17.03227
2012	16.83752
2013	17.19956
2014	16.50248
2015	16.39638
2016	16.41891
2017	16.50972
2018	16.4962