| Year | Value | | 2020 | 3.863 | | 2019 | 3.469 | | 2018 | 3.483 | | 2017 | 3.468 | | 2016 | 3.456 | | 2015 | 3.417 | | 2014 | 3.361 | | 2013 | 3.335 | | 2012 | 3.283 | | 2011 | 3.294 | | 2010 | 3.244 | | 2009 | 3.28 | | 2008 | 3.343 | | 2007 | 3.26 | | 2006 | 3.183 | | 2005 | 3.142 | | 2004 | 3.024 | | 2003 | 2.996 | | 2002 | 2.965 | | 2001 | 2.783 | | 2000 | 2.942 | | 1999 | 2.993 | | 1998 | 2.947 | | 1997 | 3.189 | | 1996 | 3.107 | | 1995 | 3.156 | | 1994 | 3.171 | | 1993 | 3.185 | | 1992 | 3.249 | | 1991 | 3.526 | | 1990 | 3.5 | | 1987 | 3.484 | | 1986 | 3.408 | | 1985 | 3.35 | | 1980 | 2.938 |
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