| Year | Value | | 2020 | 1.6800510735526 | | 2019 | 1.6726006189756 | | 2018 | 1.6995882675405 | | 2017 | 2.3314867574431 | | 2016 | 4.2412172178477 | | 2015 | 4.2937769615593 | | 2014 | 4.4203091041896 | | 2013 | 4.54927362648 | | 2012 | 4.6341137963784 | | 2011 | 4.8019832581261 | | 2010 | 4.9316259319937 | | 2009 | 5.1136921099718 | | 2008 | 6.0378482699576 | | 2007 | 6.5311703035899 | | 2006 | 6.8037049636491 | | 2005 | 7.1673665936339 | | 2004 | 7.3830887890258 | | 2003 | 7.6913578390565 | | 2002 | 7.0721421644905 | | 2001 | 6.9588889559505 | | 2000 | 7.8190110087963 | | 1999 | 8.623840010139 | | 1998 | 8.7747912158749 | | 1997 | 9.0262501610019 | | 1996 | 3.4686090877558 | | 1995 | 3.6477102682209 | | 1994 | 3.2614437363099 | | 1993 | 3.4110795518572 | | 1992 | 3.5364184795548 | | 1991 | 3.6825557989403 |
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