Year	Number
1981	18.12277
1986	16.06452
1989	15.66625
1990	15.526
1991	15.02561
1992	14.46436
1993	11.90954
1994	11.64172
1995	11.14672
1999	9.36143
2000	9.56728
2001	9.74147
2002	9.78774
2003	9.59743
2004	9.28847
2005	9.23904
2006	9.27489
2007	9.2711
2008	9.17491
2009	8.91879
2010	8.69691
2011	9.2596
2012	9.6539
2013	9.9033
2014	9.97808
2015	10.76957
2016	10.86654
2017	11.57645
2018	11.58494