| Year | Value | | 2021 | 7.6546690163368 | | 2020 | 7.224706364156 | | 2019 | 7.0707492142888 | | 2018 | 7.05088778364 | | 2017 | 7.0440247983555 | | 2016 | 7.0176746543755 | | 2015 | 7.0234506408583 | | 2014 | 7.0489280631576 | | 2013 | 7.0598849400571 | | 2012 | 7.0644165521208 | | 2011 | 7.0645313457262 | | 2010 | 7.044319852373 | | 2009 | 7.0182637186901 | | 2008 | 7.0831483592856 | | 2007 | 6.9060083833436 | | 2006 | 6.8228333133044 | | 2005 | 6.6237470125475 | | 2004 | 6.645776074163 | | 2003 | 6.5330731070314 | | 2002 | 6.588828814477 | | 2001 | 6.6156923681083 | | 2000 | 6.6413484631166 | | 1999 | 6.6472839019552 | | 1998 | 6.6732260978488 | | 1997 | 6.6925232746987 | | 1996 | 6.7550081635539 | | 1995 | 6.7775648523666 | | 1994 | 6.6937943667014 | | 1993 | 6.8089111028668 | | 1992 | 6.8548435117672 | | 1991 | 6.941429261362 | | 1990 | 6.9555934509545 | | 1989 | 6.8998569665239 | | 1988 | 7.0292145026365 | | 1987 | 7.1125194868866 | | 1986 | 7.262147460305 | | 1985 | 7.2581689310078 | | 1984 | 7.3497862265742 | | 1983 | 7.4559497866763 | | 1982 | 7.2850045402688 | | 1981 | 7.1687769764637 | | 1980 | 7.1938953872223 | | 1979 | 7.1657642991191 | | 1978 | 7.2641344209405 | | 1977 | 7.8001874799205 | | 1976 | 8.4122563415595 | | 1975 | 8.5769209674813 | | 1974 | 8.4283195156062 | | 1973 | 8.2576822985608 | | 1972 | 8.8191218821188 | | 1971 | 8.6014393105553 | | 1970 | 8.9026039534672 | | 1969 | 9.2641933328844 | | 1968 | 9.5272942963347 | | 1967 | 9.6880506663191 | | 1966 | 10.205006714972 | | 1965 | 11.177810071864 | | 1964 | 12.207471524127 | | 1963 | 11.255278971225 | | 1962 | 11.293730034504 | | 1961 | 14.477375709998 | | 1960 | 22.919699532084 |
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