Year	Number
1971	8.22551
1972	8.36996
1973	8.42986
1974	8.71112
1975	8.90986
1976	8.86321
1977	9.03523
1978	8.63733
1979	7.87554
1980	7.60969
1981	7.28337
1982	7.40909
1983	7.1775
1984	6.91012
1985	6.87425
1986	6.68983
1987	6.51876
1988	6.47138
1989	6.286
1990	6.18102
1991	5.91764
1992	6.12685
1993	6.62008
1994	7.7288
1996	10.75296
1997	11.12913
1998	12.91809
1999	13.10308
2000	14.45108
2001	14.45149
2002	14.80433
2003	16.40697
2004	17.08665
2005	17.15705
2006	17.75259
2007	18.40078
2008	17.50508
2009	16.74861
2010	15.81355
2011	16.8272
2012	15.68257
2013	15.166
2014	15.12788
2015	14.62243
2016	13.60645