| Year | Value | | 2020 | 4.4263480581334 | | 2019 | 3.9106781555873 | | 2018 | 3.8017916893784 | | 2017 | 3.7034825234974 | | 2016 | 3.6149973587926 | | 2015 | 3.5356490656163 | | 2014 | 3.6287531513453 | | 2013 | 4.060808578052 | | 2012 | 4.5136767225742 | | 2011 | 4.6998830904081 | | 2010 | 4.7648765348806 | | 2009 | 4.8868155446452 | | 2008 | 4.8399936315873 | | 2007 | 5.0098846342224 | | 2006 | 5.1254625561356 | | 2005 | 5.8543533315747 | | 2004 | 5.9726326943319 | | 2003 | 6.1218793522174 | | 2002 | 6.2501105562009 | | 2001 | 6.3968860441521 | | 2000 | 5.9194232039844 | | 1999 | 6.4303723416094 | | 1998 | 5.5769525271129 | | 1997 | 6.1774298870101 | | 1996 | 5.8168637265901 | | 1995 | 5.5188268450501 | | 1994 | 4.5088590061753 | | 1993 | 3.419109867089 | | 1992 | 3.5003968693207 | | 1991 | 3.4693578604085 |
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