Year	Number
1970	22.52724
1971	19.73143
1972	20.03889
1973	19.26352
1974	18.57297
1975	20.0465
1976	19.34911
1978	19.80863
1979	18.98551
1980	14.26407
1981	13.95354
1982	13.7474
1983	13.51929
1984	13.24036
1986	12.60917
1987	12.42025
1988	13.26617
1991	13.30215
2001	11.73818
2002	11.74075
2003	11.49056
2004	11.52665
2005	11.32837
2006	11.10342
2007	11.0839
2008	11.90842
2010	9.29398
2011	8.99283
2012	8.37204
2013	8.14088
2014	8.16851
2015	8.45773
2016	8.30333