| Year | Value | | 2020 | 5.5189043701232 | | 2019 | 5.5331627984184 | | 2018 | 5.83947310152 | | 2017 | 5.7605728668156 | | 2016 | 5.929989905437 | | 2015 | 5.9963419998611 | | 2014 | 6.1234815420771 | | 2013 | 6.255132106458 | | 2012 | 6.3887203319175 | | 2011 | 6.5222537282612 | | 2010 | 6.6567629112928 | | 2009 | 6.7711002122831 | | 2008 | 6.9144154925457 | | 2007 | 6.4885908943441 | | 2005 | 7.2460132992213 | | 2004 | 6.1954727990583 | | 2003 | 6.986929329766 | | 2002 | 7.1126223024079 | | 2001 | 7.5202778921735 | | 2000 | 7.7864293659622 | | 1999 | 8.0497294396494 | | 1998 | 8.3911613100867 | | 1997 | 8.8358426659 | | 1996 | 9.2346254479893 | | 1995 | 9.4674911899735 | | 1994 | 6.0794893228969 | | 1993 | 6.1959788097525 | | 1992 | 6.0694343287206 | | 1991 | 2.2581689260901 |
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