| Year | Value | | 2021 | 31.019274746815 | | 2020 | 30.977035899781 | | 2019 | 30.939858220901 | | 2018 | 30.894601038639 | | 2017 | 30.853391684901 | | 2016 | 30.814200333127 | | 2015 | 30.765211143408 | | 2014 | 30.705770926549 | | 2013 | 30.644329055224 | | 2012 | 30.583893932467 | | 2011 | 30.523462756751 | | 2010 | 30.45905707196 | | 2009 | 30.37090244221 | | 2008 | 30.280770486451 | | 2007 | 30.190650300339 | | 2006 | 30.102507182032 | | 2005 | 30.013384258806 | | 2004 | 29.925244017889 | | 2003 | 29.845872518286 | | 2002 | 29.752848607529 | | 2001 | 29.664696855921 | | 2000 | 29.576545104313 | | 1999 | 29.518756733814 | | 1998 | 29.460968363316 | | 1997 | 29.403179992817 | | 1996 | 29.345391622319 | | 1995 | 29.28760325182 | | 1994 | 29.229814881322 | | 1993 | 29.172026510823 | | 1992 | 29.114238140324 | | 1991 | 29.056449769826 | | 1990 | 28.998661399327 |
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