| Year | Value | | 2018 | 5.0565449420118 | | 2017 | 4.7270500277153 | | 2016 | 4.7083984506587 | | 2015 | 4.7284718202168 | | 2014 | 4.7858654112537 | | 2013 | 4.7967702991446 | | 2012 | 4.8330719747075 | | 2011 | 4.8462168902825 | | 2010 | 4.8848353330451 | | 2009 | 5.0538590438559 | | 2008 | 5.1167717794891 | | 2007 | 5.0351586638678 | | 2006 | 5.0504398968894 | | 2005 | 5.2458806746551 | | 2004 | 5.1793112266609 | | 2003 | 5.2419623141123 | | 2002 | 5.4384959674726 | | 2001 | 5.4813664202841 | | 2000 | 5.5401906365877 | | 1999 | 5.8517461921635 | | 1998 | 5.9617867395751 | | 1997 | 6.0773124548928 | | 1996 | 6.0093537638404 | | 1995 | 6.3184876453772 | | 1994 | 6.5390894118232 | | 1993 | 6.4031317421249 | | 1992 | 5.5302664309952 | | 1991 | 5.8016685808513 | | 1990 | 5.5648840506616 | | 1985 | 7.9874144938473 | | 1980 | 7.5564558534114 | | 1970 | 8.6705184012973 | | 1960 | 8.5701573238748 |
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