| Year | Value | | 2021 | 6.6352110939908 | | 2020 | 6.6267303543914 | | 2019 | 6.6215531587057 | | 2018 | 6.6114206471495 | | 2017 | 6.6012881355932 | | 2016 | 6.570691814632 | | 2015 | 6.5644907782608 | | 2014 | 6.5644907782608 | | 2013 | 6.5644907782608 | | 2012 | 6.5644907782608 | | 2011 | 6.5644907782608 | | 2010 | 6.5644907782608 | | 2009 | 6.480603649402 | | 2008 | 6.3967165205432 | | 2007 | 6.3128293916845 | | 2006 | 6.2289422628257 | | 2005 | 6.1450551339669 | | 2004 | 6.0611680051082 | | 2003 | 5.9772808762494 | | 2002 | 5.8933937473907 | | 2001 | 5.8095066185319 | | 2000 | 5.7256194896731 | | 1999 | 5.7102949483042 | | 1998 | 5.6949704069353 | | 1997 | 5.6796458655664 | | 1996 | 5.6643213241975 | | 1995 | 5.6489967828287 | | 1994 | 5.6336722414598 | | 1993 | 5.6183477000909 | | 1992 | 5.603023158722 | | 1991 | 5.5876986173531 | | 1990 | 5.5723740759842 |
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