| Year | Value | | 2022 | 2.4442092898006 | | 2021 | 2.3784945186722 | | 2020 | 2.7322694978289 | | 2019 | 3.3350367940246 | | 2018 | 3.4816074772388 | | 2017 | 3.6336548426539 | | 2016 | 3.7845661509348 | | 2015 | 3.9207854774794 | | 2014 | 4.0367432369581 | | 2013 | 4.1280454669089 | | 2012 | 4.1959363944413 | | 2011 | 4.4725181364733 | | 2010 | 4.756088762362 | | 2009 | 4.7800365914605 | | 2008 | 4.7500807389997 | | 2007 | 4.6816068095262 | | 2006 | 4.5984021773844 | | 2005 | 4.5388230472664 | | 2004 | 4.5395396828875 | | 2003 | 4.6084190172683 | | 2002 | 4.7308067483687 | | 2001 | 4.8851547732011 | | 2000 | 4.470575159187 | | 1999 | 3.9382001461856 | | 1998 | 3.9127344754859 | | 1997 | 3.8715663771461 | | 1996 | 3.8197159493861 | | 1995 | 3.770301534851 | | 1994 | 3.7271769861461 | | 1993 | 3.7010009144278 | | 1992 | 3.6961939324553 | | 1991 | 3.7093251968233 | | 1990 | 3.7487061098914 | | 1989 | 3.8261180787904 | | 1988 | 3.9747373697792 | | 1987 | 4.3090485507026 | | 1986 | 4.8734914434639 | | 1985 | 5.5377318986243 | | 1984 | 6.1196022062917 | | 1983 | 6.4400740498384 | | 1982 | 6.1358482799552 | | 1981 | 4.9853365975762 | | 1980 | 3.085606219465 | | 1979 | 1.1230112644509 | | 1978 | 0.18847643999843 | | 1977 | -1.6891867670218 | | 1976 | -2.5971398695166 | | 1975 | -2.5430947717011 | | 1974 | -2.5458045536609 | | 1973 | -2.4236700395909 | | 1972 | -2.1351371758258 | | 1971 | -1.5910478144532 | | 1970 | 0.594841963406 | | 1969 | 1.1104374625438 | | 1968 | 2.229052093402 | | 1967 | 2.2486932718604 | | 1966 | 2.1817145250262 | | 1965 | 2.1094807858065 | | 1964 | 2.0403946351449 | | 1963 | 1.9725603359717 | | 1962 | 1.9012712686053 | | 1961 | 1.8209782012155 |
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