Year	Number
1998	17.92681
1999	18.08005
2000	17.69554
2001	17.60682
2002	17.57703
2003	17.73988
2004	17.6114
2005	17.72563
2006	18.06623
2007	17.8566
2008	17.47949
2009	17.53254
2010	17.60081
2011	17.96153
2012	17.81729
2013	17.54875
2014	17.56356
2015	17.27475
2016	16.57925
2017	16.60443
2018	16.77369