Year	Number
1971	23.45107
1972	23.36051
1973	23.52003
1974	23.56364
1975	23.78632
1976	23.65994
1977	23.64108
1978	23.66661
1979	23.82273
1980	24.14228
1981	23.76956
1982	24.12887
1983	24.38624
1984	24.38101
1985	24.44807
1986	23.94556
1987	23.80446
1988	22.8871
1989	22.48901
1990	22.01148
1991	22.22887
1992	22.84509
1993	23.20199
1994	24.00922
1995	24.83965
1996	25.66189
1997	26.51097
1998	27.38462
1999	28.03208
2000	28.15927
2001	27.98211
2002	27.34991
2003	26.80241
2004	25.92522
2005	24.97141
2006	25.50149
2007	25.13212
2008	25.68853
2009	25.43502
2010	25.33858
2011	25.65008
2012	25.86966
2013	25.6577
2014	25.62664
2015	24.95432
2016	24.75304
2017	24.31458
2018	24.74825