| Year | Value | | 2020 | 12.284637835624 | | 2019 | 14.497553574477 | | 2018 | 15.804023626789 | | 2017 | 11.859194017587 | | 2016 | 12.979473203421 | | 2015 | 13.262038619355 | | 2014 | 13.698650698679 | | 2013 | 14.722697912683 | | 2012 | 15.054241716148 | | 2011 | 13.835224701687 | | 2010 | 13.46153263551 | | 2009 | 9.4656185060839 | | 2008 | 8.625153926065 | | 2007 | 7.6982860701182 | | 2006 | 9.265712755665 | | 2005 | 8.5762682559156 | | 2004 | 7.909603281409 | | 2003 | 9.6443472274353 | | 2002 | 9.5368537421892 | | 2001 | 8.8029856362038 | | 2000 | 10.364649343798 | | 1999 | 7.3625064712756 | | 1998 | 8.3114486989984 | | 1997 | 6.793272607048 | | 1996 | 7.8524970289433 | | 1995 | 6.8982550376609 | | 1994 | 6.1949356321201 | | 1993 | 6.4884725920735 | | 1992 | 5.2711099521736 | | 1991 | 5.1529067435318 | | 1990 | 4.6000563663271 | | 1989 | 5.4113938147745 | | 1988 | 6.224939944719 | | 1986 | 7.1119929270911 | | 1985 | 6.7287936014849 | | 1984 | 7.6035812449192 | | 1983 | 6.6349007003469 | | 1982 | 5.7203642936667 |
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