| Year | Value | | 2022 | 3.0952092542498 | | 2021 | 3.15796436945 | | 2020 | 3.1426036588533 | | 2019 | 3.1266699137683 | | 2018 | 3.1748500891853 | | 2017 | 3.2164072580797 | | 2016 | 3.1904414229907 | | 2015 | 3.1467471669995 | | 2014 | 3.170675517928 | | 2013 | 2.9200445018792 | | 2012 | 2.9181087435986 | | 2011 | 3.2348119628506 | | 2010 | 3.2497394697786 | | 2009 | 3.2555744003116 | | 2008 | 3.2847639070798 | | 2007 | 3.3020869112754 | | 2006 | 3.3056918279916 | | 2005 | 3.3054604014412 | | 2004 | 3.2666281158956 | | 2003 | 3.206760104409 | | 2002 | 3.1344205591604 | | 2001 | 3.0210783097382 | | 2000 | 2.907829296426 | | 1999 | 2.7536824352238 | | 1998 | 2.4898978231301 | | 1997 | 2.2195308952719 | | 1996 | 2.1026257085669 | | 1995 | 2.1321865352447 | | 1994 | 2.1364505042957 | | 1993 | 2.0826298364575 | | 1992 | 2.0245621930473 | | 1991 | 1.9807283175533 | | 1990 | 1.929091624238 | | 1989 | 1.801470507784 | | 1988 | 1.6750526197213 | | 1987 | 1.6627391223714 | | 1986 | 1.7787041327675 | | 1985 | 1.9430186889601 | | 1984 | 2.090859488827 | | 1983 | 2.1542201193471 | | 1982 | 2.147074578366 | | 1981 | 2.1507466362276 | | 1980 | 2.1178997401344 | | 1979 | 2.0312898734169 | | 1978 | 1.9646449801616 | | 1977 | 1.8582816428399 | | 1976 | 1.7837527584556 | | 1975 | 1.7805986588282 | | 1974 | 1.7508334280108 | | 1973 | 1.688768393861 | | 1972 | 1.5765267566689 | | 1971 | 1.5206722272757 | | 1970 | 1.503431699367 | | 1969 | 1.4623967135256 | | 1968 | 1.4376092864933 | | 1967 | 1.4010407308265 | | 1966 | 1.3906461834908 | | 1965 | 1.3892196894258 | | 1964 | 1.3713725950094 | | 1963 | 1.3667614847786 | | 1962 | 1.3668993627514 | | 1961 | 1.3644631476882 |
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