Year	Number
1974	17.79768
1975	17.85816
1976	18.73677
1977	20.02616
1978	19.11723
1979	19.12631
1980	19.51639
1981	20.13976
1982	19.80937
1983	19.75686
1984	19.64091
1985	19.48121
1986	19.52207
1987	19.5596
1988	19.73105
1989	19.81511
1990	19.37131
1991	19.1706
1992	19.54673
1993	20.1457
1994	20.23011
1995	20.36644
1996	20.44087
1997	20.20404
1998	20.34908
1999	19.26571
2000	18.15247
2001	18.51677
2002	18.08163
2003	17.59661
2004	17.48539
2005	17.06288
2006	17.90456
2007	17.88953
2008	17.94568
2009	18.39153
2010	18.64315
2011	18.85468
2012	19.2271
2013	19.70604
2014	18.98863
2015	18.84935
2016	18.85184
2017	18.67112
2018	18.29623