| Year | Value | | 2020 | 2.3924199596579 | | 2019 | 2.264574724538 | | 2018 | 2.2719312229795 | | 2017 | 2.0035018222614 | | 2016 | 2.0820747197816 | | 2015 | 2.1751673468669 | | 2014 | 2.271927622439 | | 2013 | 2.2854801820249 | | 2012 | 2.3113299918262 | | 2011 | 2.3707557051204 | | 2010 | 2.3097467167853 | | 2009 | 2.3072640499779 | | 2008 | 2.4064947489045 | | 2007 | 2.3383824673316 | | 2006 | 2.4464124845489 | | 2005 | 2.4749035263568 | | 2004 | 2.0238919565518 | | 2003 | 2.6596410566772 | | 2002 | 2.6581547488457 | | 2001 | 2.7226741248572 | | 2000 | 3.8890572310667 | | 1999 | 4.2592843697438 | | 1998 | 4.667256486462 | | 1997 | 5.185739383976 | | 1996 | 5.3921866028968 | | 1995 | 4.8659165757416 | | 1994 | 3.426339635527 | | 1993 | 3.4160208791337 | | 1992 | 3.3983702883575 | | 1991 | 3.0437355340741 |
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