Year	Number
1970	19.42301
1971	17.12772
1972	16.07683
1973	15.57464
1974	15.95343
1975	18.68798
1976	15.61802
1977	15.02817
1978	14.42004
1979	13.23746
1980	12.34324
1981	11.61834
1984	11.05164
1985	10.8093
1986	10.03709
1991	11.83195
1992	11.93772
1993	12.07103
1994	11.95649
1998	10.61521
1999	11.01632
2000	10.85358
2001	10.95232
2002	11.20533
2003	10.63001
2004	10.19265
2005	10.09195
2006	10.78425
2007	10.80323
2008	10.54877
2009	10.45838
2011	9.93749
2012	10.09168
2013	9.75452
2014	9.14518
2015	8.83654
2016	8.56453
2017	8.71672
2018	8.26843