Year	Number
1997	15.7941
1998	16.03762
1999	14.64726
2000	15.4094
2001	16.98514
2002	17.73842
2003	16.51283
2004	16.25008
2005	15.77308
2006	16.14508
2007	16.4558
2008	15.54546
2009	15.49385
2010	15.33663
2011	15.32361
2012	15.24673
2013	15.53086
2014	14.34632
2015	14.22179
2016	14.43956
2017	14.62196
2018	14.5885