| Year | Value | | 2020 | 2.2612721441025 | | 2019 | 2.1980361125765 | | 2018 | 2.2310042191543 | | 2017 | 2.2698085595675 | | 2016 | 2.4265579661803 | | 2015 | 2.5319947330346 | | 2014 | 2.6204801430626 | | 2013 | 2.692479045429 | | 2012 | 2.7837779844074 | | 2011 | 2.8404902692043 | | 2010 | 2.8090536405104 | | 2009 | 2.9278259312698 | | 2008 | 3.112498812336 | | 2007 | 3.4032431115673 | | 2006 | 3.7352237497322 | | 2005 | 4.548664208944 | | 2004 | 5.0333881413375 | | 2003 | 5.6088777526747 | | 2002 | 5.8030570171176 | | 2001 | 6.1329105867923 | | 2000 | 6.3919341027775 | | 1999 | 6.6498559983902 | | 1998 | 6.8386665092297 | | 1997 | 7.1341807667961 | | 1996 | 7.241801246446 | | 1995 | 7.4306074150031 | | 1994 | 3.09222961351 | | 1993 | 2.8120581051549 | | 1992 | 2.9186124082722 | | 1991 | 3.4556190525557 |
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