| Year | Value | | 2022 | 6.6990341407983 | | 2021 | 5.1314074717637 | | 2020 | 6.6234367762858 | | 2019 | 3.7295057353913 | | 2018 | 3.938826466916 | | 2017 | 3.328173374613 | | 2016 | 4.9482163406217 | | 2015 | 4.9069734412722 | | 2014 | 6.6656567186787 | | 2013 | 10.017878474611 | | 2012 | 9.4789969141979 | | 2011 | 8.9117933648337 | | 2010 | 11.989389920424 | | 2009 | 10.882352941177 | | 2008 | 8.3492670490757 | | 2007 | 6.3728813559324 | | 2006 | 5.7965233756162 | | 2005 | 4.2463436203193 | | 2004 | 3.7672517347751 | | 2003 | 3.8058589952885 | | 2002 | 4.2971520392957 | | 2001 | 3.7792931223563 | | 2000 | 4.0094359104519 | | 1999 | 4.6698203803759 | | 1998 | 13.230838976798 | | 1997 | 7.1642521146271 | | 1996 | 8.9771523382646 | | 1995 | 10.224886163755 | | 1994 | 10.247935555612 | | 1993 | 6.3268904877986 | | 1992 | 11.787817041813 | | 1991 | 13.870246177368 | | 1990 | 8.9712325027328 | | 1989 | 7.0742800294228 | | 1988 | 9.3834718618532 | | 1987 | 8.801125812518 | | 1986 | 8.7297207270266 | | 1985 | 5.556424232365 | | 1984 | 8.3189071186004 | | 1983 | 11.868081298674 | | 1982 | 7.890742793775 | | 1981 | 13.112546897642 | | 1980 | 11.346073479509 | | 1979 | 6.2756833676233 | | 1978 | 2.5230487569518 | | 1977 | 8.3074700917116 | | 1976 | -7.6339476339287 | | 1975 | 5.7484302976744 | | 1974 | 28.598734077509 | | 1973 | 16.940815979174 | | 1972 | 6.4420974615822 | | 1971 | 3.0799386840655 | | 1970 | 5.0922616199754 | | 1969 | 0.58413661046617 | | 1968 | 3.2374124262748 | | 1967 | 13.062202477797 | | 1966 | 10.801848348934 | | 1965 | 9.4747585917073 | | 1964 | 13.355261150815 | | 1963 | 2.9461613568524 | | 1962 | 3.6322149710698 | | 1961 | 1.6952129391446 | | 1960 | 1.7798778467738 |
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