| Year | Value | | 2020 | 4.2448617498608 | | 2019 | 4.2155650503132 | | 2018 | 4.2577474812725 | | 2017 | 4.3503785073705 | | 2016 | 4.5833294000335 | | 2015 | 4.6734026145218 | | 2014 | 4.7565963291967 | | 2013 | 4.8839344234522 | | 2012 | 4.9601040949759 | | 2011 | 5.2265040253002 | | 2010 | 5.2991439248064 | | 2009 | 5.4220311625035 | | 2008 | 5.5654714540952 | | 2007 | 5.6640085627563 | | 2006 | 5.8162657373217 | | 2005 | 5.6750120513395 | | 2004 | 5.8218847293287 | | 2003 | 5.9448851397235 | | 2002 | 5.8996783196045 | | 2001 | 6.0237663045616 | | 2000 | 6.5035740832753 | | 1999 | 6.7228935963081 | | 1998 | 7.0115336921524 | | 1997 | 7.1275065852177 | | 1996 | 7.282222931724 | | 1995 | 7.2925482931711 | | 1994 | 7.8616920146032 | | 1993 | 8.0353555644837 | | 1992 | 8.4826943661787 | | 1991 | 9.3341599796417 |
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