| Year | Value | | 2021 | 5.485 | | 2015 | 2.716 | | 2014 | 2.563 | | 2013 | 2.395 | | 2012 | 2.312 | | 2011 | 2.207 | | 2010 | 2.13 | | 2009 | 1.899 | | 2008 | 3.324 | | 2003 | 3.082 | | 2002 | 3.217 | | 2001 | 3.036 | | 2000 | 2.565 | | 1998 | 2.507 | | 1997 | 2.379 | | 1993 | 2.425 | | 1992 | 2.353 | | 1991 | 2.373 | | 1990 | 2.189 | | 1987 | 2.032 | | 1984 | 1.38 | | 1982 | 1.266 | | 1981 | 1.172 | | 1975 | 1.276 | | 1971 | 1.026 | | 1970 | 1.072 | | 1960 | 0.891 |
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