Year	Number
1973	26.50146
1974	24.56794
1975	21.42208
1976	20.24993
1977	19.28108
1978	19.12179
1979	18.75178
1980	18.03352
1981	18.04188
1982	18.13329
1983	17.91428
1984	17.42439
1985	17.02051
1986	16.84294
1987	16.35778
1990	14.85702
1991	14.13632
1992	14.13189
1993	13.21125
1994	12.15885
2000	13.23902
2001	12.94736
2002	11.06542
2003	11.11103
2004	11.57131
2005	10.52502
2006	10.60093
2007	11.72552
2008	11.26325
2009	11.24954
2010	10.82935
2011	11.10043
2012	11.73422
2013	12.78162
2014	13.40675
2015	13.30619
2016	12.7774
2017	12.35346