Year	Number
1981	27.81818
1986	19.13966
1991	21.3276
1996	19.65719
1999	24.4244
2000	24.14416
2001	24.41889
2002	24.03802
2003	24.45576
2004	24.23079
2005	24.48841
2006	23.84174
2007	24.2314
2008	24.19711
2009	24.03166
2010	24.28436
2011	24.91639
2012	23.89988
2013	24.54686
2014	25.30667
2015	26.19366
2016	25.03207
2017	24.91353
2018	24.98946