Year	Number
1997	18.72869
1998	18.84742
1999	20.21553
2000	19.98196
2001	19.93647
2002	19.47481
2003	18.29127
2004	18.25125
2005	18.09784
2006	19.19101
2007	19.98157
2008	19.42348
2009	19.4416
2010	19.15546
2011	18.78515
2012	18.50492
2013	18.96925
2014	18.69286
2015	18.58316
2016	18.39945
2017	18.44099
2018	18.41552