Year	Number
1981	7.7278
1986	7.20612
1989	6.83486
1990	6.32849
1991	6.11845
1992	6.07786
1993	6.35897
1994	5.93559
1995	6.50995
1996	6.8312
1997	7.02856
1998	7.48868
1999	7.35163
2000	8.08269
2002	8.00914
2003	8.13036
2004	7.93538
2005	7.82728
2006	8.05745
2007	7.14934
2008	6.86284
2009	7.31326
2010	7.35266
2011	7.28173
2012	7.11051
2015	9.4955
2016	9.26066
2017	9.67055