Year	Number
1970	20.59747
1971	21.83943
1972	23.00333
1973	21.2371
1974	19.93684
1975	20.09417
1976	20.95925
1977	21.02231
1978	20.9199
1979	20.22543
1980	18.88137
1981	17.90215
1982	16.66647
1983	16.38346
1984	16.41878
1985	17.07836
1986	16.9997
1987	16.89917
1988	16.35938
1989	15.31392
1990	14.60696
1991	14.42533
1992	14.52494
1993	14.58926
1994	14.48036
1995	15.07365
1996	15.56384
1997	16.18689
2000	17.10846
2001	18.9229
2002	18.92909
2003	18.61115
2006	17.54954
2007	16.36853
2008	15.99176
2009	15.71505
2010	15.46166
2011	15.15325
2012	14.50326
2013	15.14167
2014	14.27963
2015	13.81544
2016	13.49871
2017	13.30917
2018	13.25879