Year	Number
1997	17.76167
1998	22.55639
1999	23.1356
2000	23.39205
2001	22.43142
2002	21.44841
2003	21.09962
2004	19.83256
2005	18.83834
2006	19.35799
2007	18.93642
2008	19.54132
2009	22.30476
2010	19.78894
2011	20.18235
2012	19.88672
2013	20.25448
2014	20.40431
2015	19.8814
2016	19.7269
2017	19.8743
2018	19.74912