Year	Number
1970	20.6119
1971	20.75471
1972	20.66393
1973	20.52852
1974	20.49087
1975	20.48397
1976	20.5429
1977	20.46702
1978	20.43272
1979	20.46023
1980	20.15384
1981	20.05778
1982	19.92858
1983	19.85404
1984	19.77321
1985	19.56115
1986	19.36426
1987	19.33449
1988	19.19001
1989	19.03838
1990	18.91203
1991	18.73202
1992	18.6062
1993	18.45598
1994	18.10317
1995	18.16781
1996	18.09793
1997	17.97541
1998	18.04873
1999	17.8465
2000	17.51876
2001	17.50334
2002	17.20469
2003	17.13826
2004	16.93225
2005	16.71032
2006	16.49507
2007	16.30766
2008	16.17271
2009	16.10044
2010	15.84508
2011	15.96349
2012	15.99823
2013	15.95834
2014	15.86232
2015	15.65422
2016	15.41332
2017	15.33363
2018	15.32034