Year	Number
1970	16.1797
1971	16.19498
1972	16.39712
1973	17.20228
1974	17.98456
1975	18.29574
1976	17.82099
1977	17.57937
1978	17.64838
1979	17.72625
1980	17.83809
1981	17.63313
1982	16.76253
1983	16.22069
1984	15.66025
1985	15.57469
1986	15.36158
1987	15.20374
1988	14.87451
1989	14.50521
1990	13.78747
1991	12.5028
1992	12.37052
1993	12.74146
1994	12.63377
1995	12.44464
1996	12.38734
1997	12.48128
1998	12.79742
1999	12.89095
2000	12.82541
2001	12.27545
2002	12.65827
2003	13.08264
2004	13.59023
2005	13.86725
2006	13.734
2007	13.89214
2008	13.70683
2009	13.61116
2010	13.78848
2011	13.43101
2012	13.58114
2013	13.47809
2014	13.57558
2015	13.2923
2016	13.10002
2017	12.5853
2018	12.61647