Year	Number
1973	17.55435
1974	17.60934
1975	15.21851
1976	15.69937
1977	15.78546
1978	22.86616
1979	16.35426
1980	16.1286
1991	10.38605
1993	12.572
2000	9.79384
2001	9.72101
2002	8.8972
2003	8.77611
2004	7.74783
2005	7.13761
2006	7.06038
2007	7.31822
2008	7.25845
2009	7.27779
2010	7.34729
2011	7.52542
2012	8.16457
2013	9.7729
2014	9.8942
2015	9.9287
2016	9.64497
2017	9.51599