| Year | Value | | 2021 | 5.7364737925439 | | 2020 | 5.7788279699329 | | 2019 | 5.6433826838261 | | 2018 | 5.7262801621869 | | 2017 | 5.7237395218738 | | 2016 | 5.6152678571532 | | 2015 | 5.5514063755599 | | 2014 | 5.5426071919758 | | 2013 | 5.603189227925 | | 2012 | 5.6196339195346 | | 2011 | 5.5943191056204 | | 2010 | 5.5938674689587 | | 2009 | 5.5882097490872 | | 2008 | 5.6184227891889 | | 2007 | 5.8784780348654 | | 2006 | 5.8492296044002 | | 2005 | 5.8057864873908 | | 2004 | 5.8467232634576 | | 2003 | 5.8994319109303 | | 2002 | 5.8270972641562 | | 2001 | 5.7995443347473 | | 2000 | 5.7351769870524 | | 1999 | 5.952127363995 | | 1998 | 6.0280275807139 | | 1997 | 6.0124335488173 | | 1996 | 6.1180770370645 | | 1995 | 6.1382832904218 | | 1994 | 6.1457688888066 | | 1993 | 6.2150587178395 | | 1992 | 6.0996142589883 | | 1991 | 5.9859781058403 | | 1990 | 5.7591642432437 | | 1989 | 5.748004149147 | | 1988 | 5.7750013892233 | | 1987 | 5.7653925945135 | | 1986 | 5.7108424250283 | | 1985 | 5.6717126345231 | | 1984 | 5.6399410969307 | | 1983 | 5.5809430974123 | | 1982 | 5.6039347435494 | | 1981 | 5.5284837090411 | | 1980 | 5.4378831941023 | | 1979 | 5.715264785967 | | 1978 | 5.6188295330357 | | 1977 | 5.6915322207198 | | 1976 | 5.7653925945135 | | 1975 | 5.7818317372701 | | 1974 | 5.8533767388445 | | 1973 | 5.8793089077 | | 1972 | 5.8042639697336 | | 1971 | 5.6831705634201 | | 1970 | 5.6110470527763 | | 1969 | 5.5920610884977 | | 1968 | 5.5790950641124 | | 1967 | 5.6013225344873 | | 1966 | 5.5802527448611 | | 1965 | 5.6066478659313 | | 1964 | 5.566129039727 | | 1963 | 5.5351031956619 | | 1962 | 5.5220214032017 | | 1961 | 5.5327878341645 |
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