Year	Number
1997	18.60755
1998	18.70515
1999	20.05953
2000	19.84704
2001	19.75573
2002	19.32386
2003	18.20684
2004	18.12595
2005	17.94148
2006	18.97205
2007	19.72002
2008	19.18753
2009	19.21527
2010	18.95791
2011	18.61315
2012	18.31305
2013	18.70272
2014	18.47498
2015	18.40132
2016	18.22945
2017	18.27038
2018	18.25184