Year	Number
1981	24.10745
1986	24.09567
1989	24.14035
1990	24.10282
1991	23.048
1992	20.82443
1993	20.18343
1994	20.22664
1995	20.1038
1996	23.10311
1997	22.40959
1998	21.19301
1999	20.90977
2000	21.44349
2001	21.71928
2002	21.29445
2003	21.09375
2004	20.14552
2005	19.94866
2006	19.38332
2007	18.24208
2008	17.60546
2009	17.11574
2010	17.76213
2011	15.6244
2012	14.97123
2013	15.64401
2014	18.12079
2015	19.03192
2016	20.42443
2017	21.15014
2018	21.50664