| Year | Value | | 2021 | 30.367333451294 | | 2020 | 30.346427943161 | | 2019 | 30.29716009012 | | 2018 | 30.247480086724 | | 2017 | 30.197766192015 | | 2016 | 30.19550187642 | | 2015 | 30.06539888636 | | 2014 | 30.028836360418 | | 2013 | 29.992933607592 | | 2012 | 29.955363743809 | | 2011 | 29.918836363908 | | 2010 | 29.882208128482 | | 2009 | 29.761707569151 | | 2008 | 29.640531405896 | | 2007 | 29.519485616029 | | 2006 | 29.398036192071 | | 2005 | 29.282625003014 | | 2004 | 29.166477072014 | | 2003 | 29.04466912753 | | 2002 | 28.935871956495 | | 2001 | 28.813695610237 | | 2000 | 28.692550009825 | | 1999 | 28.79819654892 | | 1998 | 28.81032208451 | | 1997 | 28.822447620101 | | 1996 | 28.834573155691 | | 1995 | 28.846680431175 | | 1994 | 28.858787697665 | | 1993 | 28.87091321791 | | 1992 | 28.883038738154 | | 1991 | 28.895164252071 | | 1990 | 28.902708383382 |
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