Year	Number
1998	17.10322
1999	17.52523
2000	17.2198
2001	17.24665
2002	17.27979
2003	17.10656
2004	17.08376
2005	17.1981
2006	17.45775
2007	17.20407
2008	17.01331
2009	16.82876
2010	16.70653
2011	16.92272
2012	16.89228
2013	16.52313
2014	16.47877
2015	16.15424
2016	15.25996
2017	15.49078
2018	15.51576