| Year | Value | | 2021 | 2.0272249256945 | | 2020 | 2.0266257759001 | | 2019 | 2.0218776169996 | | 2018 | 2.0223709405729 | | 2017 | 2.0127008978752 | | 2016 | 1.997513867278 | | 2015 | 1.9453376591541 | | 2014 | 2.0014428329433 | | 2013 | 1.9307729065822 | | 2012 | 2.0031564103333 | | 2011 | 2.0333567764091 | | 2010 | 2.0155378645438 | | 2009 | 2.0066904082013 | | 2008 | 1.9713261361973 | | 2007 | 1.9197195810037 | | 2006 | 1.9183350815585 | | 2005 | 1.8927467633701 | | 2004 | 1.8602404535724 | | 2003 | 1.8502766801279 | | 2002 | 1.8905325455679 | | 2001 | 1.9179949925258 | | 2000 | 2.0383517941574 | | 1999 | 1.9893194248439 | | 1998 | 1.9880103992112 | | 1997 | 2.0205454762507 | | 1996 | 2.0357967282197 | | 1995 | 2.0342867283443 | | 1994 | 2.0571348468837 | | 1993 | 2.0580574666387 | | 1992 | 1.9979097756778 | | 1991 | 1.6033870169312 | | 1990 | 1.631779634394 | | 1989 | 1.6091947790624 | | 1988 | 1.6139859423742 | | 1987 | 1.5964193363345 | | 1986 | 1.5688896695054 | | 1985 | 1.5853452025129 | | 1984 | 1.5863787578731 | | 1983 | 1.5655441699775 | | 1982 | 1.5667001624607 | | 1981 | 1.5552084939785 | | 1980 | 1.5593427562183 | | 1979 | 1.5640316113619 | | 1978 | 1.5611485902093 | | 1977 | 1.532236906522 | | 1976 | 1.5502421318372 | | 1975 | 1.5435241666748 | | 1974 | 1.5672353043895 | | 1973 | 1.5915926697794 | | 1972 | 1.5769048112529 | | 1971 | 1.5630329448667 | | 1970 | 1.5583409900596 | | 1969 | 1.54793709027 | | 1968 | 1.5450811177787 | | 1967 | 1.5426331413576 | | 1966 | 1.5283532789012 | | 1965 | 1.5185613732168 | | 1964 | 1.5185613732168 | | 1963 | 1.5112174439536 | | 1962 | 1.5112174439536 | | 1961 | 1.5046895068307 |
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