Year	Number
1970	13.73929
1971	14.39169
1972	14.94854
1973	14.9384
1974	14.63953
1975	14.83745
1976	15.51352
1977	16.14466
1978	16.48359
1979	16.16815
1980	15.67693
1981	15.35551
1982	14.86255
1983	14.78281
1984	14.82242
1985	14.9843
1986	15.08604
1987	15.0741
1988	14.89063
1989	14.47574
1990	14.14092
1991	14.08039
1992	14.21873
1993	14.31692
1994	14.38821
1995	14.71883
1996	14.97421
1997	15.28674
1998	15.40402
1999	15.37203
2000	15.82149
2001	16.64336
2002	16.76972
2003	16.46943
2004	16.37975
2005	16.25185
2006	16.09261
2007	15.59635
2008	15.27202
2009	15.10351
2010	14.91824
2011	14.67161
2012	14.28352
2013	14.80778
2014	14.3671
2015	14.02361
2016	13.75482
2017	13.58041
2018	13.50771