| Year | Value | | 2021 | 9.4182179006885 | | 2020 | 9.3799568919857 | | 2019 | 9.3482137528462 | | 2018 | 9.3187288690461 | | 2017 | 9.2527461081052 | | 2016 | 9.1542772140469 | | 2015 | 9.0384804035685 | | 2014 | 8.9576831328555 | | 2013 | 8.8808032544384 | | 2012 | 8.7486632522353 | | 2011 | 8.5249412299876 | | 2010 | 8.3691211158457 | | 2009 | 8.2026476695133 | | 2008 | 7.9855606432612 | | 2007 | 7.869067641935 | | 2006 | 7.7194186567025 | | 2005 | 7.6274593918823 | | 2004 | 7.4616253165527 | | 2003 | 7.4298490082114 | | 2002 | 7.1750092822515 | | 2001 | 7.1070333407346 | | 2000 | 7.0733414878362 | | 1999 | 7.0490889969385 | | 1998 | 7.0033638282067 | | 1997 | 6.9908282853718 | | 1996 | 6.9558350914809 | | 1995 | 6.8376690925733 | | 1994 | 6.7080258427942 | | 1993 | 6.7025256269416 | | 1992 | 6.56021195117 | | 1991 | 6.5564719519429 | | 1990 | 6.5063271548679 | | 1989 | 6.4280702786086 | | 1988 | 6.4089183038145 | | 1987 | 6.3486909059013 | | 1986 | 6.3381811603971 | | 1985 | 6.3113306480172 | | 1984 | 6.2364511946988 | | 1983 | 6.2331929749206 | | 1982 | 6.2093225416061 | | 1981 | 6.189226875108 | | 1980 | 6.1965777368028 | | 1979 | 6.1845580845721 | | 1978 | 6.1863461320113 | | 1977 | 6.1168606218014 | | 1976 | 6.1107514597172 | | 1975 | 6.0801063133273 | | 1974 | 6.0493409835079 | | 1973 | 6.0723154999574 | | 1972 | 6.0850299987673 | | 1971 | 6.0403802548999 | | 1970 | 6.0428635554487 | | 1969 | 6.0249837914973 | | 1968 | 5.9933465425056 | | 1967 | 5.9256021035344 | | 1966 | 5.9005704340025 | | 1965 | 5.8776247369316 | | 1964 | 5.8294983722958 | | 1963 | 5.7891199053723 | | 1962 | 5.7019063900985 | | 1961 | 5.6638622256909 |
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