| Year | Value | | 2022 | 2.3116032390751 | | 2021 | 2.3717515483557 | | 2020 | 2.3967070500013 | | 2019 | 2.3907943661815 | | 2018 | 2.4350384154402 | | 2017 | 2.4788078523926 | | 2016 | 2.4429969895953 | | 2015 | 2.385955684598 | | 2014 | 2.3775340228914 | | 2013 | 2.3792915199656 | | 2012 | 2.4598404063847 | | 2011 | 2.5655096090179 | | 2010 | 2.6071051025344 | | 2009 | 2.6405702962824 | | 2008 | 2.6815986646922 | | 2007 | 2.6937067262308 | | 2006 | 2.6648071914004 | | 2005 | 2.6316930163195 | | 2004 | 2.5792097778125 | | 2003 | 2.3104045116277 | | 2002 | 1.7879819759358 | | 2001 | 1.3953022786345 | | 2000 | 1.2457312574553 | | 1999 | 1.1924358422243 | | 1998 | 3.182439181556 | | 1997 | 13.247067250558 | | 1996 | 16.625501977996 | | 1995 | -16.880629670864 | | 1994 | -16.049152615798 | | 1993 | 3.1815153097002 | | 1992 | 2.265543075103 | | 1991 | 2.2386858993997 | | 1990 | 2.4615927974137 | | 1989 | 2.6675770202777 | | 1988 | 3.1872547893953 | | 1987 | 3.5976616967172 | | 1986 | 3.6206075568516 | | 1985 | 3.408831271975 | | 1984 | 3.3123358772293 | | 1983 | 3.5877838789899 | | 1982 | 3.802829517593 | | 1981 | 3.6382521802457 | | 1980 | 3.355470327008 | | 1979 | 3.0497764606513 | | 1978 | 2.8584753050798 | | 1977 | 2.8639096066702 | | 1976 | 2.8908939129347 | | 1975 | 2.9070836928786 | | 1974 | 2.9353751599527 | | 1973 | 2.9520683327297 | | 1972 | 2.9489626896344 | | 1971 | 3.0093135438167 | | 1970 | 3.0881839483836 | | 1969 | 3.1150564041818 | | 1968 | 3.1216416022898 | | 1967 | 3.0156076866048 | | 1966 | 2.8088078510224 | | 1965 | 2.5708468696276 | | 1964 | 2.2411332743558 | | 1963 | 2.1918264723912 | | 1962 | 2.4469600193828 | | 1961 | 2.6775078000387 |
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