Year	Number
1970	28.105
1971	28.23694
1972	27.836
1973	28.36033
1974	28.48107
1975	28.62487
1976	28.63334
1977	28.73086
1978	28.57155
1979	28.60349
1980	28.34571
1981	28.16444
1982	27.80012
1983	27.42866
1984	27.23172
1985	27.14291
1986	26.93052
1987	26.83477
1988	26.6785
1989	26.25206
1990	26.21584
1991	26.15986
1992	26.12866
1993	26.13342
1994	26.20764
1995	26.29711
1996	26.31457
1997	26.47088
1998	26.61422
1999	26.55149
2000	26.30914
2001	26.0185
2002	25.82453
2003	25.25962
2004	25.39072
2005	25.24855
2006	25.14194
2007	25.04689
2008	24.97171
2009	24.68045
2010	24.31042
2011	24.11758
2012	24.19696
2013	23.55038
2014	23.29472
2015	23.10596
2016	23.44392
2017	23.38904
2018	23.44631