| Year | Value | | 2017 | 20.775671762566 | | 2016 | 22.08912841113 | | 2015 | 20.407468165277 | | 2014 | 21.850452574837 | | 2013 | 20.784435071596 | | 2012 | 21.489920039584 | | 2011 | 21.337340503447 | | 2010 | 21.081232004066 | | 2009 | 20.85143114153 | | 2008 | 20.538554183982 | | 2007 | 21.083064721871 | | 2006 | 22.07137896493 | | 2005 | 0.15901806680626 |
|