Year	Number
1971	12.5121
1972	11.33129
1973	12.06029
1974	11.91675
1975	12.81693
1976	14.87728
1977	14.02385
1978	11.73727
1980	13.36737
1981	13.02492
1982	12.59407
1983	11.85789
1984	11.96005
1985	11.75977
1986	11.48823
1987	11.41988
1988	10.91394
1989	10.42798
1990	9.88503
1991	10.01157
1992	9.5319
1993	8.83187
1994	8.52939
1995	9.77599
1996	9.50378
1997	10.09369
1999	11.41029
2000	11.94651
2001	11.36325
2002	11.62682
2003	11.67132
2004	11.0379
2005	11.05182
2006	10.48547
2007	9.63266
2008	9.67207
2009	9.61048
2010	9.46352
2011	9.03415
2012	8.42905
2013	8.30605
2014	8.88073
2015	9.03784
2016	9.34814
2017	9.47531
2018	9.63133