| Year | Value | | 2021 | 0.541 | | 2020 | 0.509 | | 2019 | 0.548 | | 2018 | 0.518 | | 2017 | 0.788 | | 2016 | 0.433 | | 2015 | 0.453 | | 2010 | 0.383 | | 2009 | 0.333 | | 2008 | 0.311 | | 2007 | 0.292 | | 2006 | 0.284 | | 2004 | 0.289 | | 2002 | 0.295 | | 2001 | 0.298 | | 1999 | 0.291 | | 1998 | 0.292 | | 1997 | 0.273 | | 1995 | 0.24 | | 1993 | 0.238 | | 1991 | 0.23 | | 1990 | 0.225 | | 1988 | 0.21 | | 1986 | 0.182 | | 1984 | 0.16 | | 1980 | 0.147 |
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