| Year | Value | | 2020 | 22.337399353937 | | 2019 | 21.607420010472 | | 2018 | 21.286149162861 | | 2017 | 22.480716576016 | | 2016 | 23.289839447377 | | 2015 | 24.279793885207 | | 2014 | 23.358355068588 | | 2013 | 24.258869624406 | | 2012 | 23.205787898305 | | 2011 | 26.269485789421 | | 2010 | 27.045983147532 | | 2009 | 25.654591332908 | | 2008 | 30.599540919505 |
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