| Year | Value | | 2021 | 5.1515151515151 | | 2020 | 5.1515151515151 | | 2019 | 5.1515151515151 | | 2018 | 5.1515151515151 | | 2017 | 5.1515151515151 | | 2016 | 5.1515151515151 | | 2015 | 5.1515151515151 | | 2014 | 5.1515151515151 | | 2013 | 5.1515151515151 | | 2012 | 5.1515151515151 | | 2011 | 5.1515151515151 | | 2010 | 5.1515151515151 | | 2009 | 5.0555555555556 | | 2008 | 4.959595959596 | | 2007 | 4.8636363636364 | | 2006 | 4.7676767676768 | | 2005 | 4.6717171717172 | | 2004 | 4.5757575757576 | | 2003 | 4.479797979798 | | 2002 | 4.3838383838384 | | 2001 | 4.2878787878788 | | 2000 | 4.1919191919192 | | 1999 | 4.1919191919192 | | 1998 | 4.1919191919192 | | 1997 | 4.1919191919192 | | 1996 | 4.1919191919192 | | 1995 | 4.1919191919192 | | 1994 | 4.1919191919192 | | 1993 | 4.1919191919192 | | 1992 | 4.1919191919192 | | 1991 | 4.1919191919192 | | 1990 | 4.1919191919192 |
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