Year	Number
1997	18.49814
1998	18.59869
1999	20.0017
2000	19.77718
2001	19.69439
2002	19.22253
2003	18.13605
2004	18.07503
2005	17.88727
2006	18.95101
2007	19.76356
2008	19.22322
2009	19.27835
2010	19.01667
2011	18.69427
2012	18.40884
2013	18.87957
2014	18.61942
2015	18.59749
2016	18.43681
2017	18.41274
2018	18.39302