Year	Number
1998	20.13219
1999	19.57774
2000	21.21829
2001	19.75146
2002	19.6739
2003	19.37623
2004	19.62125
2005	18.86241
2006	18.62464
2007	17.99788
2008	17.98088
2009	21.9377
2010	17.79208
2011	17.94183
2012	17.6913
2013	17.31922
2014	17.21654
2015	16.80643
2016	16.74685
2017	16.77519