Year	Number
1970	21.77235
1971	21.79851
1972	22.31014
1973	21.87968
1974	22.36075
1975	22.45594
1976	21.92858
1977	22.08145
1978	21.8334
1979	21.18811
1980	20.48414
1981	19.94867
1982	19.81772
1983	18.35798
1984	18.08844
1985	17.77079
1986	17.79843
1987	17.61955
1988	17.39261
1989	17.69419
1990	18.38983
1991	17.77533
1992	17.95424
1993	18.15854
1994	18.65248
1995	18.64583
1996	18.27505
1997	18.18119
1998	18.4072
1999	18.62204
2000	18.52779
2001	18.3493
2002	18.32992
2003	18.08882
2004	17.98254
2005	17.47877
2006	16.96247
2007	16.34881
2008	15.9296
2009	15.50535
2010	15.32184
2011	15.17937
2012	15.05347
2013	14.63249
2014	14.68665
2015	14.79831
2016	14.91463
2017	14.99075
2018	14.91591