Year	Number
1970	15.68715
1971	16.08391
1972	16.40472
1973	16.40298
1974	16.30168
1975	16.56045
1976	17.03808
1977	17.46
1978	17.71883
1979	17.49367
1980	17.31738
1981	17.18005
1982	17.02283
1983	17.06654
1984	17.20164
1985	17.51305
1986	17.42429
1987	17.35504
1988	17.20579
1989	17.00694
1990	16.85824
1991	16.99035
1992	17.28259
1993	17.5725
1994	17.79326
1995	18.14505
1996	18.3458
1997	18.64655
1998	18.82496
1999	18.84449
2000	19.21638
2001	19.8228
2002	19.80609
2003	19.66473
2004	19.75225
2005	19.43027
2006	19.13407
2007	18.72028
2008	18.3246
2009	18.09796
2010	18.09872
2011	18.23437
2012	18.35552
2013	19.03864
2014	18.92542
2015	18.6707
2016	18.27281
2017	18.07721
2018	18.11664