Year	Number
1970	19.38946
1971	19.48105
1972	19.27581
1973	19.33391
1974	19.57926
1975	19.84218
1976	20.20377
1977	20.50004
1978	20.60865
1979	20.81698
1980	21.12245
1981	21.39043
1982	21.66084
1983	21.77941
1984	21.95471
1985	22.01991
1986	21.88257
1987	21.6576
1988	21.38123
1989	21.45859
1990	21.28406
1991	21.64452
1992	22.13574
1993	22.61878
1994	23.068
1995	23.377
1996	23.60982
1997	23.85023
1998	24.35032
1999	24.57848
2000	24.84319
2001	24.71771
2002	24.35929
2003	24.48463
2004	24.73769
2005	23.62931
2006	22.90208
2007	22.44207
2008	21.62776
2009	21.14691
2010	21.20589
2011	21.70244
2012	22.38171
2013	23.50979
2014	23.80539
2015	23.48068
2016	22.55303
2017	22.27369
2018	22.34035