Year	Number
1981	15.55255
1986	13.86609
1989	9.14587
1990	9.01603
1991	11.86914
1992	11.84313
1993	9.95819
1994	8.46538
1995	7.77949
2006	7.54696
2007	7.3501
2008	7.61973
2009	6.97771
2010	7.00329
2011	7.08139
2012	7.42693
2013	7.35359
2014	9.16601
2015	9.70719
2016	10.05152
2017	10.02216
2018	9.92033