Year	Number
1971	19.97828
1972	19.45624
1973	17.18566
1974	16.86167
1975	16.03441
1976	20.43812
1993	9.5482
1994	10.16069
1995	10.52424
1996	11.21408
1997	11.83295
1998	12.4508
1999	12.3228
2000	11.83778
2001	11.40012
2002	11.46596
2003	11.18875
2004	10.10989
2005	10.05038
2006	9.96625
2007	9.8046
2008	9.55123
2009	9.34009
2010	9.53888
2011	9.2663
2012	9.64567
2013	11.80923
2014	11.93004
2015	12.09754
2016	12.3537
2017	12.23325