Year	Number
1970	27.06892
1971	26.43547
1972	25.87193
1973	26.81489
1974	27.07162
1975	26.85665
1976	28.38767
1977	27.2216
1978	26.35474
1979	24.77946
1980	27.3054
1981	26.61049
1982	26.19933
1983	25.99528
1984	25.58287
1985	25.46111
1986	22.24628
1987	22.50858
1988	23.04419
1989	23.25568
1990	23.01928
1999	26.08202
2000	24.73639
2001	24.3295
2002	24.27752
2004	24.03741
2005	24.23255
2006	24.5048
2007	24.53809
2008	24.23802
2009	23.62485
2010	23.45812
2011	23.0112
2012	22.60998
2013	22.34957
2014	21.98706
2015	21.89367
2016	21.44301
2017	21.96162