Year	Number
1981	17.9017
1982	17.49132
1999	14.25064
2000	14.23481
2001	17.34958
2002	17.19698
2003	17.36837
2004	14.45002
2005	14.57447
2006	14.22194
2007	14.25017
2008	14.21478
2009	13.94637
2010	14.1419
2011	14.23097
2012	14.33889
2013	12.04695
2014	12.32599
2016	12.17537
2017	12.48274