| Year | Value | | 2019 | 2.434 | | 2018 | 2.368 | | 2017 | 2.322 | | 2016 | 2.27 | | 2015 | 2.205 | | 2014 | 2.106 | | 2013 | 1.999 | | 2012 | 1.9 | | 2011 | 1.826 | | 2010 | 1.749 | | 2009 | 1.661 | | 2008 | 1.621 | | 2007 | 1.583 | | 2006 | 1.545 | | 2005 | 1.553 | | 2004 | 1.52 | | 2003 | 1.489 | | 2002 | 1.443 | | 2001 | 1.437 | | 2000 | 1.376 | | 1999 | 1.343 | | 1998 | 1.627 | | 1994 | 1.411 | | 1993 | 1.385 | | 1992 | 1.348 | | 1991 | 1.32 | | 1990 | 1.266 | | 1988 | 1.111 | | 1986 | 0.723 | | 1985 | 0.998 | | 1981 | 0.877 | | 1980 | 0.851 | | 1975 | 0.667 | | 1970 | 0.657 | | 1965 | 0.526 | | 1960 | 0.42 |
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