Year	Number
1970	29.57253
1971	29.78266
1972	28.38822
1973	29.25362
1974	29.52529
1975	29.78746
1976	29.57729
1977	29.68792
1978	29.42964
1979	29.34114
1980	28.64126
1981	28.01928
1982	27.19006
1983	26.59563
1984	26.0968
1985	26.02852
1986	25.59199
1987	25.18456
1988	24.59958
1989	24.01249
1990	23.53229
1991	23.41908
1992	23.50576
1993	23.57333
1994	23.78656
1995	23.99273
1996	24.19947
1997	24.43232
1998	24.62919
1999	24.49215
2000	23.94191
2001	23.24656
2002	22.39397
2003	20.3403
2004	20.81492
2005	20.18158
2006	19.59351
2007	19.08614
2008	19.20127
2009	18.74053
2010	18.3924
2011	18.32389
2012	18.30572
2013	17.79425
2014	17.4832
2015	17.24798
2016	17.0038
2017	17.50504
2018	17.64651