| Year | Value | | 2020 | 0.14209429946089 | | 2019 | 0.14536723861186 | | 2018 | 0.1526139430006 | | 2017 | 0.15459215287421 | | 2016 | 0.1660570897023 | | 2015 | 0.17133700927285 | | 2014 | 0.17666533306198 | | 2013 | 0.17956618378262 | | 2012 | 0.18253557052329 | | 2011 | 0.18562220644659 | | 2010 | 0.18857753740135 | | 2009 | 0.19151202001359 | | 2008 | 0.1911220170525 | | 2007 | 0.19427226645106 | | 2005 | 0.20206675674952 | | 2004 | 0.18829900780201 | | 2003 | 0.28215250004044 | | 2002 | 0.27757644918038 | | 2001 | 0.28068116971308 | | 2000 | 0.22903257451261 | | 1999 | 0.21394404179345 | | 1998 | 0.2180189121303 | | 1997 | 0.21719082777071 | | 1996 | 0.2277263207187 | | 1995 | 0.23178397373595 | | 1994 | 0.2062113138983 | | 1993 | 0.20977632249986 | | 1992 | 0.21312343043474 | | 1991 | 0.24025518945203 |
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