Year	Number
1970	10.01846
1971	11.49797
1972	11.94645
1973	13.13854
1974	14.80597
1975	11.77585
1976	12.33817
1977	12.34791
1978	12.0033
1979	13.06841
1980	12.83604
1981	11.8072
1982	11.77728
1983	12.05732
1986	12.38101
1987	12.13049
1988	11.98218
1989	11.86375
1990	11.69909
1991	12.16292
1992	11.83884
1993	12.04711
1994	12.13061
1996	11.38528
1998	13.34528
1999	14.13379
2000	15.18242
2001	15.36702
2002	14.77309
2003	13.33185
2004	13.62689
2005	15.63157
2007	14.34258
2008	15.87981
2009	17.94259
2010	18.97493
2011	19.38717
2012	19.96351
2013	20.53221
2014	18.95573
2015	19.31742
2016	19.46537
2017	19.2394