| Year | Value | | 2021 | 7.3803738764412 | | 2020 | 4.1957725571678 | | 2019 | 14.363758080297 | | 2018 | 9.1394871375418 | | 2017 | 4.5444741013859 | | 2016 | 6.8663464518119 | | 2015 | 5.9861519052041 | | 2014 | 4.1319465097838 | | 2013 | 5.7145977929233 | | 2012 | 6.1045226818591 | | 2011 | 5.617863295665 | | 2010 | 6.1528707029565 | | 2009 | 5.0352613790531 | | 2008 | 5.247023658311 | | 2007 | 7.5068666505787 | | 2006 | 8.0232018536618 | | 2005 | 6.586093018704 | | 2004 | 7.7229049475592 | | 2003 | 4.1605330690639 | | 2002 | 3.7210067637051 | | 2001 | 4.8038030274726 | | 2000 | 5.6731865959988 | | 1999 | 4.4468597727143 | | 1998 | 4.1529668475534 | | 1997 | 2.1561830773605 | | 1996 | 2.8615125890298 | | 1995 | 3.7579389696848 | | 1994 | 5.1899625164731 | | 1993 | 3.0221184079306 | | 1992 | 3.2608852871225 | | 1991 | -1.4179860610023 | | 1990 | 7.0218818509247 | | 1989 | 2.5201522899011 | | 1988 | 2.5684981199948 | | 1987 | 3.6251625203097 | | 1986 | 3.9284561885006 | | 1985 | 9.1960295091826 | | 1984 | 2.2873550921712 | | 1983 | 2.3297761042812 | | 1982 | -2.1629450630681 | | 1981 | 11.964826193313 | | 1980 | 1.9431492883396 | | 1979 | 6.1661880186694 | | 1978 | 9.0397527838273 | | 1977 | -2.5199414806087 | | 1976 | 0.79447519050578 | | 1975 | -4.6828309507455 | | 1974 | -1.0477473609502 |
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