Year	Number
1970	29.46948
1971	29.68587
1972	28.31758
1973	29.17493
1974	29.44075
1975	29.69483
1976	29.48375
1977	29.58827
1978	29.33112
1979	29.24473
1980	28.55898
1981	27.94907
1982	27.13263
1983	26.54786
1984	26.05759
1985	25.99351
1986	25.56358
1987	25.16123
1988	24.58104
1989	23.99779
1990	23.52047
1991	23.40749
1992	23.49294
1993	23.5597
1994	23.77117
1995	23.97595
1996	24.18134
1997	24.41248
1998	24.60798
1999	24.47194
2000	23.92607
2001	23.23621
2002	22.39064
2003	20.35435
2004	20.82463
2005	20.19631
2006	19.61266
2007	19.1086
2008	19.22298
2009	18.76529
2010	18.41768
2011	18.34781
2012	18.32777
2013	17.81751
2014	17.50663
2015	17.27039
2016	17.02758
2017	17.52599
2018	17.66661