Year	Number
1998	18.58074
1999	18.3465
2000	18.76493
2001	18.27472
2002	17.97954
2003	18.02213
2004	17.92955
2005	17.83709
2006	18.1956
2007	17.57669
2008	17.876
2009	17.34231
2010	17.69822
2011	17.84675
2012	17.31711
2013	17.22761
2014	17.48331
2015	17.11589
2016	16.71746
2017	17.16156