Year	Number
1973	15.54008
1974	15.65119
1975	15.57366
1976	15.45163
1977	15.19171
1978	14.97718
1979	14.73674
1980	14.57938
1981	14.61774
1982	14.31095
1983	14.02345
1984	13.75389
1985	13.45206
1986	13.13523
1987	12.98835
1988	12.81997
1989	12.89946
1990	12.77289
1991	12.35718
1992	12.18963
1993	12.40316
1994	12.13213
1995	12.18708
1996	12.30283
1997	12.37925
1998	12.3857
1999	12.28572
2000	12.29466
2001	12.20403
2002	12.2811
2003	11.89632
2004	11.85311
2005	11.99818
2006	11.90761
2007	11.82012
2008	11.67794
2009	11.61264
2010	11.56469
2011	11.60414
2012	11.54085
2013	11.33605
2014	11.27
2015	11.39267
2016	11.4289
2017	11.39754
2018	11.35679