| Year | Value | | 2022 | 8.2906180734014 | | 2021 | 4.1208159652656 | | 2020 | 2.7324920941943 | | 2019 | 2.7264303738882 | | 2018 | 3.2380155894772 | | 2017 | 3.3969758489525 | | 2016 | 2.7246122329529 | | 2015 | 3.1290027447393 | | 2014 | 3.5892201661666 | | 2013 | 4.4706251432132 | | 2012 | 4.5372050816697 | | 2011 | 5.7991117314592 | | 2010 | 4.1232285607532 | | 2009 | 3.687001604988 | | 2008 | 10.252529154714 | | 2007 | 6.2143973324648 | | 2006 | 5.577752076543 | | 2005 | 5.038838071119 | | 2004 | 4.7230860202747 | | 2003 | 4.6342267436203 | | 2002 | 4.2971520392957 | | 2001 | 5.3263613829784 | | 2000 | 4.3666645129168 | | 1999 | 4.861922674901 | | 1998 | 7.695765175172 | | 1997 | 7.1451116088393 | | 1996 | 9.3303145853193 | | 1995 | 11.128702970709 | | 1994 | 13.651896803889 | | 1993 | 10.748646716372 | | 1992 | 10.098967153517 | | 1991 | 13.441558441559 | | 1990 | 11.396346119956 | | 1989 | 9.3043612584317 | | 1988 | 8.9104702001338 | | 1987 | 8.2258064516129 | | 1986 | 9.0295105435716 | | 1985 | 8.5083743439159 | | 1984 | 10.45503482319 | | 1983 | 10.671936758893 | | 1982 | 10.258808855628 | | 1981 | 12.974697469819 | | 1978 | 8.9337588409501 | | 1977 | 11.26579363707 |
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