Year	Number
1981	13.01437
1991	18.63712
1996	18.89025
1999	19.44182
2000	19.03178
2001	14.76885
2002	9.06057
2003	9.10834
2004	7.4692
2005	5.86031
2006	6.29894
2007	6.71748
2008	6.64168
2009	6.27066
2010	6.24063
2011	6.21254
2012	5.8358
2014	8.86705
2016	8.71211
2017	8.72723