| Year | Value | | 2020 | 1.883 | | 2019 | 1.651 | | 2018 | 2.163 | | 2016 | 1.219 | | 2013 | 1.285 | | 2012 | 1.278 | | 2011 | 1.234 | | 2010 | 1.249 | | 2009 | 1.248 | | 2007 | 1.218 | | 2006 | 1.207 | | 2004 | 1.208 | | 2003 | 1.196 | | 2002 | 1.17 | | 2001 | 1.3 | | 2000 | 1.359 | | 1998 | 1.338 | | 1997 | 1.323 | | 1996 | 1.308 | | 1995 | 1.292 | | 1994 | 1.285 | | 1993 | 1.368 | | 1992 | 1.593 | | 1991 | 1.452 | | 1990 | 1.371 | | 1985 | 1.406 | | 1984 | 1.407 | | 1983 | 1.415 | | 1980 | 1.367 | | 1977 | 1.036 | | 1972 | 0.845 | | 1971 | 0.911 | | 1970 | 0.739 | | 1965 | 0.481 | | 1960 | 0.276 |
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