Year	Number
1970	25.62988
1971	25.9696
1972	26.27675
1975	27.34629
1976	27.69524
1977	27.17693
1979	26.56297
1980	22.75733
1981	22.05917
1982	21.1878
1983	22.20474
1984	22.81973
1985	22.0875
1986	21.94277
1987	21.28795
1988	20.92586
1989	20.35318
1990	19.3354
1991	18.75422
1992	18.17791
1993	17.96686
1994	18.60713
1995	18.68252
1996	18.95773
1997	18.59158
1998	19.32134
2000	18.37824
2001	17.89326
2002	17.71513
2003	17.74779
2004	17.36274
2005	16.26544
2006	16.29535
2007	14.97044
2008	14.22366
2009	13.65341
2010	13.72496
2011	13.57944
2012	13.92335
2013	13.27612
2016	13.21255
2017	12.2751
2018	11.40208