Year	Number
1970	23.47983
1971	21.11807
1972	20.73884
1973	20.15193
1974	19.41098
1975	19.03224
1976	18.35855
1977	18.27825
1978	16.55962
1979	18.06518
1980	18.26032
1981	17.59833
1982	16.4753
1983	16.00146
1984	14.74603
1985	16.08083
1986	16.62157
1988	13.99853
1989	14.02604
1991	15.30987
1992	15.53378
1993	15.54573
1994	15.24892
1995	14.59371
1998	14.01143
1999	13.96313
2000	13.66667
2001	13.63795
2002	12.69287
2003	12.24954
2004	10.88013
2005	10.11697
2006	12.51317
2007	12.67144
2008	12.55216
2009	11.94999
2010	11.34882
2011	11.33566
2012	10.58767
2013	10.15195
2014	10.25924
2015	10.01661
2016	10.19678
2017	10.18252
2018	9.89521