Year	Number
1971	23.73294
1972	25.10335
1973	25.47666
1974	25.50434
1975	24.25782
1976	24.04392
1977	23.57156
1978	23.89312
1979	23.79699
1980	24.13942
1981	24.29661
1982	23.89145
1983	25.822
1984	25.98868
1985	26.09922
1986	25.78402
1987	25.50322
1988	26.4562
1989	26.64064
1990	26.71172
1991	20.32429
1992	20.43253
1993	20.63443
1994	19.09058
1995	19.02811
1996	17.93365
1997	16.83159
1998	17.01465
1999	17.09145
2000	17.30293
2001	17.9595
2002	17.78794
2003	18.35494
2004	18.40998
2005	18.27565
2006	18.13279
2007	17.75029
2008	17.41255
2009	17.06677
2010	17.38226
2011	18.037
2012	18.01613
2013	16.5705
2014	16.04885
2015	15.94475
2016	15.45586
2017	15.18548