| Year | Value | | 2021 | 8.8722858864498 | | 2020 | 8.648546377254 | | 2019 | 7.7886909292194 | | 2018 | 7.0852096809729 | | 2017 | 8.2006192873047 | | 2016 | 7.3241667804827 | | 2015 | 6.7830093204347 | | 2014 | 11.295118994462 | | 2013 | 15.387372139395 | | 2012 | 16.59080238465 | | 2011 | 18.329280548427 | | 2010 | 12.248013152175 | | 2009 | 8.7739546928911 | | 2008 | 7.4915250419398 | | 2007 | 8.0423332612421 | | 2006 | 8.264716998848 | | 2005 | 9.0394275801845 | | 2004 | 8.7055146100893 | | 2003 | 10.360988074388 | | 1997 | 11.745618959594 | | 1996 | 5.8901492570421 | | 1995 | 4.4109972581407 | | 1994 | 3.3289431854308 | | 1993 | 3.1631097560976 | | 1992 | 0.52910052910053 |
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