Year	Number
1971	27.05996
1972	28.5622
1973	28.82339
1974	26.1997
1975	24.24474
1976	23.17937
1977	21.61838
1978	19.63922
1979	18.75056
1980	17.91887
1981	17.47407
1982	16.96203
1983	16.3521
1984	15.37914
1985	14.75258
1986	13.96964
1987	13.29788
1988	12.68259
1989	12.29152
1990	12.319
1991	12.51003
1992	12.34485
1993	12.37419
1994	12.90747
1995	13.57722
1996	11.86058
1997	11.7956
1998	11.83972
1999	11.7748
2000	11.49998
2001	10.73922
2003	10.68011
2004	10.24083
2005	10.30837
2006	9.97784
2007	9.64855
2008	9.61042
2009	9.43833
2010	9.12865
2011	9.11033
2012	9.05528
2013	9.06109
2014	9.06511
2015	8.89917
2016	8.74431
2017	8.82731
2018	9.16029