Year	Number
1971	18.21
1972	18.3577
1973	18.19487
1974	17.99787
1975	17.14525
1976	16.05928
1977	15.55843
1978	15.97336
1979	16.43236
1980	16.60179
1981	16.24146
1982	15.37939
1983	14.58783
1984	13.91426
1985	13.25832
1986	12.65882
1987	12.04161
1988	11.75139
1989	11.62283
1990	11.5937
1991	11.62863
1992	11.88489
1993	11.90163
1994	10.51708
1995	10.60344
1997	11.26543
1998	12.07828
1999	11.75936
2000	11.5904
2001	11.01922
2002	10.4732
2003	9.61999
2004	10.40137
2005	10.57957
2006	10.65804
2007	10.68532
2008	10.79644
2009	10.87947
2010	10.91983
2011	11.1084
2012	11.19463
2013	11.13524
2014	12.82368
2015	11.92696
2016	11.79992