Year	Number
1971	9.5371
1972	9.41307
1973	8.76409
1974	13.80537
1975	13.71281
1999	17.20259
2000	16.66163
2001	16.84405
2002	16.3341
2003	16.21437
2004	16.19266
2005	16.44698
2008	21.76787
2009	20.39814
2010	19.6934
2011	18.70971
2012	19.11185
2013	18.9193
2014	19.32933
2016	19.90775
2017	19.84342