| Year | Value | | 2021 | 30.514377554444 | | 2020 | 30.495095779513 | | 2019 | 30.447528999177 | | 2018 | 30.399551166051 | | 2017 | 30.351539433917 | | 2016 | 30.350619558446 | | 2015 | 30.222821423841 | | 2014 | 30.187862360002 | | 2013 | 30.153561610876 | | 2012 | 30.11759742663 | | 2011 | 30.082673424221 | | 2010 | 30.047648779069 | | 2009 | 29.929386975495 | | 2008 | 29.810450929058 | | 2007 | 29.691645014171 | | 2006 | 29.572436139481 | | 2005 | 29.459256106723 | | 2004 | 29.345341532057 | | 2003 | 29.225775565173 | | 2002 | 29.11920403748 | | 2001 | 28.999271168692 | | 2000 | 28.880367768441 | | 1999 | 28.988695217375 | | 1998 | 29.003601181556 | | 1997 | 29.018507145737 | | 1996 | 29.033413109918 | | 1995 | 29.04830082543 | | 1994 | 29.063188528524 | | 1993 | 29.078094473983 | | 1992 | 29.093000419442 | | 1991 | 29.10790635862 | | 1990 | 29.118283693887 |
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