Year	Number
1991	22.60489
1993	20.47717
1994	19.91538
1997	20.93593
1998	20.8617
1999	20.48241
2000	19.88406
2001	19.58976
2002	19.46268
2003	19.11184
2004	18.59754
2005	18.70376
2006	17.12453
2007	16.33813
2008	15.81651
2009	15.58278
2010	15.65789
2011	15.83346
2012	16.08657
2013	16.48785
2015	12.38447
2016	12.75838
2017	12.99453
2018	12.98011