| Year | Value | | 2021 | 11.612951826193 | | 2020 | 9.9556383708142 | | 2019 | 10.435391329165 | | 2018 | 10.560949928931 | | 2017 | 10.635573972765 | | 2016 | 10.421514879466 | | 2015 | 10.358251569886 | | 2014 | 10.777815919786 | | 2013 | 11.102293890224 | | 2012 | 11.009552257887 | | 2011 | 11.017843543195 | | 2010 | 10.605021841525 | | 2009 | 10.504685571111 | | 2008 | 11.459030309688 | | 2007 | 12.255241285298 | | 2006 | 12.051056350602 | | 2005 | 11.384383515573 | | 2004 | 11.742849829848 | | 2003 | 11.858395939685 | | 2002 | 12.073596162644 | | 2001 | 11.03867220998 | | 2000 | 10.064976449687 | | 1999 | 10.247150926656 | | 1998 | 9.3719339745147 | | 1997 | 8.6976692730371 | | 1996 | 8.5296637390485 | | 1995 | 7.7147312425578 | | 1994 | 6.7652272360966 | | 1993 | 7.8198271697576 | | 1992 | 8.3139487007959 | | 1991 | 7.3256864378774 | | 1990 | 6.8652116704245 |
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