| Year | Value | | 2021 | 14.069403714565 | | 2020 | 14.010752688172 | | 2019 | 13.952101661779 | | 2018 | 13.893450635386 | | 2017 | 13.834799608993 | | 2016 | 13.7761485826 | | 2015 | 13.717497556207 | | 2014 | 13.659237536657 | | 2013 | 13.600977517107 | | 2012 | 13.542717497556 | | 2011 | 13.484457478006 | | 2010 | 13.426197458455 | | 2009 | 13.43431085044 | | 2008 | 13.442424242424 | | 2007 | 13.450537634409 | | 2006 | 13.458651026393 | | 2005 | 13.466764418377 | | 2004 | 13.474877810362 | | 2003 | 13.482991202346 | | 2002 | 13.49110459433 | | 2001 | 13.499217986315 | | 2000 | 13.507331378299 | | 1999 | 13.522189638319 | | 1998 | 13.537047898338 | | 1997 | 13.551906158358 | | 1996 | 13.566764418377 | | 1995 | 13.581622678397 | | 1994 | 13.596480938416 | | 1993 | 13.611339198436 | | 1992 | 13.626197458456 | | 1991 | 13.641055718475 | | 1990 | 13.655913978495 |
|