| Year | Value | | 2021 | 4.518 | | 2020 | 4.459 | | 2019 | 4.391 | | 2018 | 4.311 | | 2017 | 4.251 | | 2016 | 4.187 | | 2015 | 4.12 | | 2014 | 4.064 | | 2013 | 3.984 | | 2012 | 3.892 | | 2011 | 3.822 | | 2010 | 3.734 | | 2009 | 3.648 | | 2008 | 3.58 | | 2007 | 3.533 | | 2006 | 3.487 | | 2005 | 3.448 | | 2004 | 3.426 | | 2003 | 3.401 | | 2002 | 3.364 | | 2001 | 3.327 | | 2000 | 3.272 | | 1999 | 3.217 | | 1998 | 3.182 | | 1997 | 3.139 | | 1996 | 3.124 | | 1995 | 3.075 | | 1994 | 3.009 | | 1993 | 2.937 | | 1992 | 2.841 | | 1991 | 2.762 |
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