Year	Number
1970	15.49122
1971	15.25572
1972	15.15383
1973	15.33821
1974	15.48895
1975	15.72708
1976	15.81605
1977	15.84773
1978	15.62143
1979	15.43596
1980	15.3634
1981	15.12658
1982	15.09166
1983	14.99069
1984	14.95164
1985	14.94546
1986	14.94947
1987	14.94725
1988	14.97159
1989	15.06092
1990	14.80982
1991	15.07694
1992	15.23966
1993	15.42605
1994	15.38252
1995	15.04059
1996	15.37916
1997	15.71584
1998	15.68226
1999	15.76473
2000	15.63834
2001	15.66756
2002	15.7846
2003	15.7082
2004	15.59951
2005	15.76129
2006	15.68583
2007	15.53653
2008	15.35853
2009	15.24127
2010	15.02396
2011	15.02347
2012	14.82787
2013	14.66267
2014	14.70222
2015	14.44537
2016	13.76732
2017	13.69945
2018	13.96217