Year	Number
1970	13.63564
1971	14.28017
1972	14.89096
1973	14.92034
1974	14.68165
1975	14.93391
1976	15.54691
1977	16.13166
1978	16.48359
1979	16.15003
1980	15.71182
1981	15.37869
1982	14.92913
1983	14.78515
1984	14.82939
1985	14.98316
1986	15.11782
1987	15.10376
1988	14.98099
1989	14.66317
1990	14.41063
1991	14.39478
1992	14.55218
1993	14.71419
1994	14.7942
1995	15.15932
1996	15.39088
1997	15.70108
1998	15.81037
1999	15.81278
2000	16.19938
2001	17.04805
2002	17.1165
2003	16.78598
2004	16.75811
2005	16.65361
2006	16.45102
2007	15.9718
2008	15.6517
2009	15.5118
2010	15.35929
2011	15.19568
2012	14.8786
2013	15.36619
2014	14.88605
2015	14.59857
2016	14.38035
2017	14.21118
2018	14.16773