Year	Number
1970	8.31083
1971	7.64789
1972	8.41692
1973	8.7261
1974	8.41762
1975	8.32497
1976	9.92241
1977	9.5143
1978	9.66261
1979	9.84328
1980	8.72399
1981	9.1348
1982	8.7311
1983	9.06037
1984	9.09276
1985	9.05661
1986	9.65027
1987	9.81777
1988	9.5402
1989	9.17906
1990	9.29446
1991	9.42783
1992	9.83791
1993	9.81196
1994	9.48693
1996	9.68151
1997	9.71044
1999	10.18898
2000	10.89401
2001	11.01901
2002	11.8757
2004	12.70874
2005	13.0624
2006	15.04596
2007	15.53906
2009	14.21373
2013	18.05688
2014	15.52306
2015	15.36408
2016	15.66259
2018	14.8453