Year	Number
1970	24.01285
1971	24.03701
1972	24.57969
1973	23.89653
1974	24.47101
1975	24.48856
1976	23.77882
1977	24.00156
1978	23.5744
1979	22.77435
1980	21.94834
1981	21.34557
1982	21.14949
1983	19.39161
1984	19.07153
1985	18.73849
1986	18.7364
1987	18.53805
1988	18.28829
1989	18.72336
1990	19.54314
1991	18.84755
1992	19.09939
1993	19.39242
1994	20.05319
1995	20.08383
1996	19.64985
1997	19.52923
1998	19.81548
1999	20.0851
2000	19.94777
2001	19.8122
2002	19.86752
2003	19.59587
2004	19.51239
2005	18.92363
2006	18.30041
2007	17.57161
2008	17.07437
2009	16.57548
2010	16.40265
2011	16.23666
2012	16.13358
2013	15.69777
2014	15.78599
2015	15.95591
2016	16.1407
2017	16.28611
2018	16.20428