| Year | Value | | 2023 | 0.317 | | 2020 | 0.253 | | 2019 | 0.214 | | 2018 | 0.303 | | 2017 | 0.245 | | 2016 | 0.284 | | 2015 | 0.29 | | 2014 | 0.304 | | 2013 | 0.29 | | 2012 | 0.246 | | 2011 | 0.258 | | 2010 | 0.244 | | 2009 | 0.22 | | 2008 | 0.183 | | 2007 | 0.182 | | 2006 | 0.166 | | 2001 | 0.202 | | 1997 | 0.11 | | 1993 | 0.143 | | 1990 | 0.109 | | 1989 | 0.129 | | 1987 | 0.179 | | 1986 | 0.183 | | 1981 | 0.077 |
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