| Year | Value | | 2022 | 18.259346379854 | | 2021 | 17.719206547358 | | 2020 | 15.666042909392 | | 2019 | 15.557754025306 | | 2018 | 14.638503003613 | | 2017 | 13.904849079633 | | 2016 | 13.217039846545 | | 2015 | 12.612444219556 | | 2014 | 12.037376220268 | | 2013 | 11.574016345704 | | 2012 | 11.037459287385 | | 2011 | 10.459113472996 | | 2010 | 9.9505402154673 | | 2009 | 9.4091266548344 | | 2008 | 8.9038061681004 | | 2007 | 8.7278128312437 | | 2006 | 8.5039132783025 | | 2005 | 8.3365042905069 | | 2004 | 8.170362715434 | | 2003 | 8.0398574370399 | | 2002 | 7.8560491364242 | | 2001 | 7.691023964998 | | 2000 | 7.6902363339768 | | 1999 | 7.6555356868717 | | 1998 | 7.6003970402902 | | 1997 | 7.5307434528472 | | 1996 | 7.4605786544724 | | 1995 | 7.3967395698161 | | 1994 | 7.3805707384305 | | 1993 | 7.3704321370827 | | 1992 | 7.379950511426 | | 1991 | 7.3299541613332 |
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