| Year | Value | | 2022 | 3.1150221864707 | | 2021 | 3.1638875051703 | | 2020 | 3.1605330596369 | | 2019 | 3.2945827275398 | | 2018 | 3.3781277487954 | | 2017 | 3.3245949164666 | | 2016 | 3.1486485056167 | | 2015 | 3.1841008127668 | | 2014 | 3.5699854657635 | | 2013 | 3.5570193239768 | | 2012 | 3.4876294025756 | | 2011 | 3.4944495210031 | | 2010 | 3.4084909649552 | | 2009 | 3.5185542308284 | | 2008 | 3.4458860053868 | | 2007 | 3.3871898707333 | | 2006 | 3.540781451756 | | 2005 | 3.9917494743908 | | 2004 | 4.4360266669424 | | 2003 | 3.9706048657473 | | 2002 | 3.4481406209876 | | 2001 | 3.3300848498951 | | 2000 | 3.4145002414948 | | 1999 | 3.4771705246094 | | 1998 | 3.4072054047812 | | 1997 | 3.4209899074847 | | 1996 | 4.4216408368017 | | 1995 | 4.3896856724851 | | 1994 | 2.219474819994 | | 1993 | 2.2799458558143 | | 1992 | 4.2208052094333 | | 1991 | 3.6291788407997 | | 1990 | 2.7215846661894 | | 1989 | 3.3638955946381 | | 1988 | 3.3791764554986 | | 1987 | 3.5072562894657 | | 1986 | 2.986018641556 | | 1985 | 0.95132807958253 | | 1984 | 1.2553273425651 | | 1983 | 4.9861619787712 | | 1982 | 4.7307701982243 | | 1981 | 0.022568870591883 | | 1980 | 0.12002944976834 | | 1979 | 2.0528182763811 | | 1978 | 1.752330553372 | | 1977 | 1.7973246634136 | | 1976 | 1.9144384600729 | | 1975 | 2.0669732961987 | | 1974 | 2.2154036469744 | | 1973 | 2.287873413583 | | 1972 | 2.1638408643476 | | 1971 | 2.0281594011559 | | 1970 | 1.9895056870206 | | 1969 | 1.9203833325056 | | 1968 | 1.8633039582068 | | 1967 | 1.8366985093799 | | 1966 | 1.8508614915105 | | 1965 | 1.9038793873689 | | 1964 | 1.9471859706557 | | 1963 | 1.9518942922349 | | 1962 | 1.9431650942634 | | 1961 | 1.9283028415749 |
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