Year	Number
1971	27.02685
1972	27.02718
1973	27.02701
1974	27.02752
1975	27.02692
1976	27.02756
1977	27.02749
1978	29.41132
1979	29.41173
1981	29.41175
1982	29.4118
1983	29.41233
1984	29.41189
1985	29.41146
1986	29.41227
1987	29.4121
1988	29.41218
1989	25.64486
1990	24.48745
1991	23.09835
1992	22.13338
1993	19.22587
1994	19.64303
1995	20.18596
1996	19.10252
1998	17.67153
1999	18.28958
2000	16.88543
2001	17.71026
2002	17.3543
2003	16.79405
2004	16.41274
2005	16.24551
2006	15.65768
2007	18.7278
2008	18.47503
2009	18.50084
2010	18.70713
2011	18.82126
2012	18.97433
2013	18.93175