Year	Number
1970	15.2196
1971	15.65142
1972	16.01034
1973	16.00696
1974	15.89557
1975	16.1549
1976	16.68048
1977	17.15178
1978	17.46589
1979	17.22016
1980	16.96561
1981	16.79686
1982	16.62365
1983	16.63031
1984	16.76453
1985	17.0341
1986	17.0518
1987	17.0228
1988	16.93461
1989	16.74815
1990	16.59672
1991	16.71563
1992	17.00806
1993	17.30572
1994	17.50499
1995	17.78168
1996	17.93249
1997	18.17914
1998	18.28725
1999	18.23987
2000	18.60503
2001	19.22808
2002	19.22615
2003	19.09272
2004	19.12028
2005	18.80545
2006	18.46705
2007	18.02382
2008	17.57182
2009	17.26407
2010	17.21703
2011	17.44554
2012	17.38847
2013	18.27212
2014	18.22951
2015	17.94955
2016	17.4179
2017	17.26164
2018	17.23354