Year	Number
1970	20.30298
1971	20.56105
1972	20.72679
1973	20.69914
1974	20.99665
1975	21.13129
1976	20.98935
1977	21.79263
1978	21.80636
1979	21.37334
1980	20.80632
1981	20.41632
1982	20.54907
1983	19.58099
1984	19.31407
1985	19.04835
1986	18.53357
1987	17.88535
1988	17.26042
1989	17.27191
1990	17.79876
1991	16.78804
1992	16.55208
1993	16.46
1994	16.79409
1995	16.36114
1996	15.81358
1997	15.64744
1998	15.96622
1999	16.28416
2000	16.29985
2001	16.32731
2002	16.50426
2003	16.47046
2004	16.56364
2005	16.17217
2006	15.71052
2007	15.20759
2008	14.85188
2009	14.56006
2010	14.51277
2011	14.46235
2012	14.41522
2013	14.36273
2014	14.44477
2015	14.50165
2016	14.52697
2017	14.55245
2018	14.49976