| Year | Value | | 2024 | 2.9821039688069 | | 2023 | 3.0098076571157 | | 2022 | 2.8733435852614 | | 2021 | 2.7442615299726 | | 2020 | 2.8732041478573 | | 2019 | 2.9663347518457 | | 2018 | 2.9603789664274 | | 2017 | 3.0122278618955 | | 2016 | 3.0021384702168 | | 2015 | 3.0402587324785 | | 2014 | 3.0683237952253 | | 2013 | 3.0456268708589 | | 2012 | 3.0377290688838 | | 2011 | 3.0497446837342 | | 2010 | 3.1385638975593 | | 2009 | 3.1969350857355 | | 2008 | 3.2123281206168 | | 2007 | 3.2388127035726 | | 2006 | 3.2506488863994 | | 2005 | 3.0966676131407 | | 2004 | 2.9336906385253 | | 2003 | 2.9383597904305 | | 2002 | 2.9662219209082 | | 2001 | 3.0215681316732 | | 2000 | 3.0774289358609 | | 1999 | 3.1137953078142 | | 1998 | 3.1504687240223 | | 1997 | 3.1875674456765 | | 1996 | 3.3663358065725 | | 1995 | 3.5937298874085 | | 1994 | 3.6912201401111 | | 1993 | 3.7605473827668 | | 1992 | 3.8026253704844 | | 1991 | 3.8308080770016 | | 1990 | 3.8437676769055 | | 1989 | 3.8478531977258 | | 1988 | 3.8831940092725 | | 1987 | 3.8641281136893 | | 1986 | 3.8302055907128 | | 1985 | 3.8104165822889 | | 1984 | 3.7339414897598 | | 1983 | 3.6480297655722 | | 1982 | 3.5648304960125 | | 1981 | 3.4906136873354 | | 1980 | 3.401560030415 | | 1979 | 3.2795799876173 | | 1978 | 3.1680264276891 | | 1977 | 3.1015145141428 | | 1976 | 3.0335221456087 |
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