Year	Number
1971	12.96823
1972	13.09606
1973	13.40465
1974	12.59936
1975	11.78334
1976	10.51455
1977	9.28765
1978	9.73214
1979	7.51239
1980	7.26461
1981	7.03296
1982	7.02983
1983	6.72826
1984	6.95872
1985	6.99377
1986	7.46099
1987	7.20583
1988	8.03858
1989	7.86441
1990	7.60703
1991	7.96514
1992	7.76129
1993	8.89366
1994	9.74934
1995	9.18802
1996	9.87947
1997	9.98962
1999	11.04881
2000	10.61418
2001	11.49453
2002	10.2497
2003	12.06622
2004	10.90852
2005	11.27424
2006	10.91473
2007	12.06192
2008	12.36964
2009	13.1546
2010	13.76581
2011	13.81562
2012	12.41538
2013	12.10603
2014	12.31071
2016	11.79911
2017	11.24677