| Year | Value | | 2021 | 4.4384695932262 | | 2020 | 4.7101329205404 | | 2019 | 4.2141530145226 | | 2018 | 4.1903849655879 | | 2017 | 3.5221102436984 | | 2014 | 3.761106023731 | | 2013 | 3.761160110901 | | 2012 | 3.8303981719372 | | 2011 | 3.9125627639122 | | 2010 | 4.0106446701464 | | 2009 | 4.2809975999301 | | 2008 | 4.3211378100397 | | 2007 | 4.15707767975 | | 2006 | 4.337984511889 | | 2005 | 4.4536842560524 | | 2004 | 4.3565661686601 | | 2003 | 4.3757170192043 | | 2002 | 4.2952311250942 | | 2001 | 3.8948421470163 | | 2000 | 4.1435808528287 | | 1999 | 3.7737332072614 | | 1998 | 3.7020823862524 | | 1997 | 3.3560005917836 | | 1996 | 3.2654353547987 | | 1995 | 3.1054620713426 | | 1994 | 3.1557982778889 | | 1993 | 2.8651974582329 | | 1992 | 2.6117054459485 | | 1991 | 2.629793731088 |
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