Year	Number
1973	15.26094
1974	15.63605
1975	15.76252
1976	17.22432
1977	19.31779
1978	17.94812
1979	17.91044
1980	18.5033
1981	18.8942
1982	17.95664
1983	17.90247
1984	17.68013
1985	17.82234
1986	18.05776
1987	18.29978
1988	18.52491
1989	18.37795
1990	17.93244
1991	17.71656
1992	18.48425
1993	19.61357
1994	19.92376
1995	20.15574
1996	20.45168
1997	20.33951
1998	20.36605
1999	19.66029
2000	19.60062
2001	19.08844
2002	18.72466
2003	17.68017
2004	17.46351
2005	16.69472
2006	17.94394
2007	17.71406
2008	17.61776
2009	17.59494
2010	17.91082
2011	17.65032
2012	17.63587
2013	18.29695
2014	17.67284
2015	17.03006
2016	16.61274
2017	16.13971
2018	15.84522