| Year | Value | | 2023 | 19.433077976765 | | 2022 | 19.594181105283 | | 2021 | 10.415709632173 | | 2020 | 12.093255821542 | | 2019 | 21.253631312867 | | 2018 | 21.32056811196 | | 2017 | 17.21953041983 | | 2016 | 16.70391316338 | | 2015 | 15.232876280505 | | 2014 | 15.725043092066 | | 2013 | 16.247688037404 | | 2012 | 14.884187552402 | | 2011 | 11.213522041697 | | 2010 | 11.285232022476 | | 2009 | 0.6570041167702 | | 2008 | 2.9340985018089 | | 2007 | 11.012219098408 | | 2006 | 11.588688507068 | | 2005 | 19.05730828256 | | 2004 | 20.957447928727 | | 2003 | 16.353517131903 | | 2002 | 19.074706779253 | | 2001 | 18.603400566132 | | 2000 | 10.3417460063 | | 1999 | 20.448042353626 | | 1998 | 15.831736080524 | | 1997 | -2.9736558431579 |
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