| Year | Value | | 2021 | 13.221455379655 | | 2020 | 13.185632207663 | | 2019 | 13.152564791469 | | 2018 | 13.11536394825 | | 2017 | 13.07816310503 | | 2016 | 13.057495969909 | | 2015 | 13.040962261811 | | 2014 | 12.961600462944 | | 2013 | 12.882238664076 | | 2012 | 12.802876865209 | | 2011 | 12.723515066342 | | 2010 | 12.644153267474 | | 2009 | 12.600752283718 | | 2008 | 12.557351299963 | | 2007 | 12.513950316207 | | 2006 | 12.470549332451 | | 2005 | 12.427148348696 | | 2004 | 12.38374736494 | | 2003 | 12.340346381185 | | 2002 | 12.296945397429 | | 2001 | 12.253544413673 | | 2000 | 12.210143429918 | | 1999 | 12.137395114289 | | 1998 | 12.064646798661 | | 1997 | 11.991898483032 | | 1996 | 11.919150167404 | | 1995 | 11.846401851775 | | 1994 | 11.773653536147 | | 1993 | 11.700905220518 | | 1992 | 11.62815690489 | | 1991 | 11.555408589261 | | 1990 | 11.482660273633 |
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