Year	Number
1973	17.63026
1974	17.97154
1975	18.08996
1976	18.8873
1977	20.11392
1978	19.2347
1979	19.27309
1980	19.65576
1981	20.24102
1982	19.84533
1983	19.74362
1984	19.58813
1985	19.4361
1986	19.47182
1987	19.51712
1988	19.65113
1989	19.71522
1990	19.23506
1991	19.01389
1992	19.40896
1993	20.02468
1994	20.10704
1995	20.21894
1996	20.2936
1997	20.06102
1998	20.20592
1999	19.12563
2000	18.03559
2001	18.38568
2002	18.00665
2003	17.54934
2004	17.44282
2005	17.04043
2006	17.87191
2007	17.85573
2008	17.91486
2009	18.35893
2010	18.61673
2011	18.80403
2012	19.1794
2013	19.65952
2014	18.94729
2015	18.79753
2016	18.79591
2017	18.60414
2018	18.23975