Year	Number
1973	19.50343
1974	19.7771
1975	19.87997
1976	20.2779
1977	20.87687
1978	20.23921
1979	20.13672
1980	20.26904
1981	20.57577
1982	20.23722
1983	20.1027
1984	19.94209
1985	19.78017
1986	19.78676
1987	19.76547
1988	19.8256
1989	19.81587
1990	19.42176
1991	19.21229
1992	19.53277
1993	19.99917
1994	20.035
1995	20.14156
1996	20.26025
1997	20.0211
1998	20.31433
1999	19.30702
2000	18.36543
2001	18.51863
2002	17.97357
2003	17.45197
2004	17.22541
2005	16.96969
2006	17.47324
2007	17.409
2008	17.46949
2009	17.70114
2010	17.68612
2011	17.72641
2012	17.89986
2013	18.27033
2014	17.75289
2015	17.70922
2016	17.82649
2017	17.72055
2018	17.45058