| Year | Value | | 2020 | 2.1976808139013 | | 2019 | 2.2115360901269 | | 2018 | 2.28115980222 | | 2017 | 2.3487517614101 | | 2016 | 2.4293288182274 | | 2015 | 2.5103026732165 | | 2014 | 2.5962921744795 | | 2013 | 2.6895844191376 | | 2012 | 2.7863567853395 | | 2011 | 2.800086417722 | | 2010 | 2.8223456598874 | | 2009 | 2.8518506095402 | | 2008 | 2.885324950281 | | 2007 | 2.9331403705769 | | 2006 | 2.9880802194721 | | 2005 | 3.0473591521768 | | 2004 | 3.1272130733143 | | 2003 | 3.1969879045626 | | 2002 | 3.272621098486 | | 2001 | 3.6148188713802 | | 2000 | 6.4446081454476 | | 1999 | 6.5921066959094 | | 1998 | 6.8667354105304 | | 1997 | 7.1845652022709 | | 1996 | 6.284845454486 | | 1995 | 6.4883181879652 | | 1994 | 1.5060040076918 | | 1993 | 2.3393213078457 | | 1992 | 3.2571765249739 | | 1991 | 2.81108974906 |
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