| Year | Value | | 2020 | 1.9897081384903 | | 2019 | 2.1278196169904 | | 2018 | 2.1322937466102 | | 2017 | 2.2363557869921 | | 2016 | 2.2609063859586 | | 2015 | 2.2833449329895 | | 2014 | 2.3094374531677 | | 2013 | 2.4761183175311 | | 2012 | 2.5105280392293 | | 2011 | 2.5491923582673 | | 2010 | 2.5944392610297 | | 2009 | 2.6284072597286 | | 2008 | 2.7605691000964 | | 2007 | 2.7808924398224 | | 2006 | 2.8116128386368 | | 2005 | 2.8480477742137 | | 2004 | 2.8766495167105 | | 2003 | 2.9165766984065 | | 2002 | 2.9178910117632 | | 2001 | 2.959998897804 | | 2000 | 3.0064102351061 | | 1999 | 3.0398943330034 | | 1998 | 3.0739285961602 | | 1997 | 3.020416947017 | | 1996 | 3.050709876897 | | 1995 | 2.9869609601407 | | 1994 | 3.5773850879517 | | 1993 | 3.6802775105896 | | 1992 | 3.7419915767271 | | 1991 | 3.8269804777051 |
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