| Year | Value | | 2020 | 1.629 | | 2019 | 1.638 | | 2018 | 1.61 | | 2017 | 1.59 | | 2016 | 1.574 | | 2015 | 1.581 | | 2013 | 1.588 | | 2012 | 1.605 | | 2011 | 1.505 | | 2010 | 1.413 | | 2009 | 1.317 | | 2008 | 1.389 | | 2007 | 1.312 | | 2006 | 1.358 | | 2005 | 1.358 | | 2004 | 1.332 | | 2003 | 1.352 | | 2002 | 1.347 | | 2001 | 1.256 | | 1999 | 1.244 | | 1998 | 1.204 | | 1997 | 1.214 | | 1996 | 1.216 | | 1995 | 1.177 | | 1994 | 1.135 | | 1993 | 1.188 | | 1992 | 1.154 | | 1991 | 1.132 | | 1990 | 1.122 | | 1988 | 1.793 | | 1985 | 1.006 | | 1984 | 1.021 | | 1981 | 0.993 | | 1975 | 0.597 | | 1970 | 0.613 | | 1965 | 0.478 | | 1960 | 0.356 |
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