Year	Number
1970	17.16569
1971	18.32271
1972	19.32063
1973	18.70961
1974	19.36182
1975	19.45419
1976	19.75379
1977	20.26675
1978	20.39189
1979	20.34494
1980	20.35816
1981	20.21892
1982	20.14648
1983	20.18309
1984	20.28568
1985	20.15015
1986	19.94919
1987	19.90078
1988	19.88935
1989	19.8784
1991	19.85892
1992	20.58411
1993	21.08985
1994	20.94314
1995	21.0487
1996	19.15089
1999	17.91693
2000	19.19542
2002	20.92474
2004	24.64723
2005	26.17165
2006	27.36572
2007	25.2348
2009	26.65632
2010	27.14634
2011	25.58137
2012	25.39509
2013	25.49841
2014	24.90129
2015	26.47999
2016	26.18799
2017	26.01138
2018	25.91073