| Year | Value | | 2022 | 2.0776449114925 | | 2021 | 1.9757060657332 | | 2020 | 1.9221265269256 | | 2019 | 1.8764701668265 | | 2018 | 1.7369863765523 | | 2017 | 1.6835063253553 | | 2016 | 1.7388492619909 | | 2015 | 1.7442583748467 | | 2014 | 1.6869003591516 | | 2013 | 1.5619043329485 | | 2012 | 1.4720999881132 | | 2011 | 2.684015505378 | | 2010 | 2.8228496834042 | | 2009 | 1.687340968602 | | 2008 | 1.6053273629522 | | 2007 | 1.423474896803 | | 2006 | 1.220216354397 | | 2005 | 1.1633502238127 | | 2004 | 1.1537692150554 | | 2003 | 1.1636752444369 | | 2002 | 1.2238315192858 | | 2001 | 1.2659685329142 | | 2000 | 1.3837468209656 | | 1999 | 1.0779083598987 | | 1998 | 1.6094902947862 | | 1997 | 1.885238808116 | | 1996 | 1.9126859984056 | | 1995 | 1.806877907471 | | 1994 | 1.9631037245837 | | 1993 | 2.2724584154641 | | 1992 | 2.3443917035247 | | 1991 | 2.1321535121122 | | 1990 | 2.0048389331323 | | 1989 | 2.5333187946087 | | 1988 | 2.7038662638814 | | 1987 | 2.7483173775621 | | 1986 | 2.7132383584289 | | 1985 | 2.6298151320478 | | 1984 | 2.5413874828839 | | 1983 | 2.4623992265791 | | 1982 | 2.4191410000292 | | 1981 | 2.4070533363846 | | 1980 | 2.4645540961843 | | 1979 | 2.6348028170732 | | 1978 | 2.8282998427423 | | 1977 | 2.9225569215196 | | 1976 | 2.9294609485621 | | 1975 | 2.9184235866452 | | 1974 | 2.9080592908966 | | 1973 | 2.900118384885 | | 1972 | 2.8932166897235 | | 1971 | 2.9430935343705 | | 1970 | 3.1264626053358 | | 1969 | 3.3189617317911 | | 1968 | 3.4331576294104 | | 1967 | 3.5218748920631 | | 1966 | 3.6337588299617 | | 1965 | 3.740826917468 | | 1964 | 3.7968747052092 | | 1963 | 3.8671305146665 | | 1962 | 3.905388894552 | | 1961 | 3.7478633169539 |
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