| Year | Value | | 2021 | 29.680617923137 | | 2020 | 29.754491837377 | | 2019 | 29.828991737988 | | 2018 | 29.902628703424 | | 2017 | 29.969802201271 | | 2016 | 30.030892009485 | | 2015 | 30.100217558392 | | 2014 | 30.204916708887 | | 2013 | 30.276551883334 | | 2012 | 30.348110859703 | | 2011 | 30.419683367585 | | 2010 | 30.490740416022 | | 2009 | 30.566367497999 | | 2008 | 30.64220962539 | | 2007 | 30.718086057525 | | 2006 | 30.795023562612 | | 2005 | 30.871028300485 | | 2004 | 30.946997743973 | | 2003 | 31.022825386366 | | 2002 | 31.098581194329 | | 2001 | 31.174577825288 | | 2000 | 31.250189169474 | | 1999 | 31.39723381174 | | 1998 | 31.468716496886 | | 1997 | 31.54019920492 | | 1996 | 31.611681890066 | | 1995 | 31.683164552327 | | 1994 | 31.754647237474 | | 1993 | 31.82612992262 | | 1992 | 31.897612630654 | | 1991 | 34.760463847342 | | 1990 | 34.8501053507 |
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