Year	Number
1981	25.30353
1986	25.34544
1989	20.38883
1990	18.63904
1991	17.86571
1992	16.69436
1993	16.86672
1994	16.70484
1995	16.48088
1996	16.32927
1997	16.01284
1998	15.4349
1999	16.53451
2000	16.44787
2001	15.95552
2002	16.31386
2003	15.55441
2004	14.72329
2005	14.07004
2006	13.64496
2007	13.2633
2008	12.99866
2009	12.80744
2010	12.84839
2011	12.41205
2012	12.43042
2013	12.61606
2014	12.80832
2015	12.86014
2016	13.22845
2017	13.51703