| Year | Value | | 2022 | 18.420369678576 | | 2021 | 17.150630894258 | | 2020 | 13.413124498905 | | 2019 | 14.39660845192 | | 2018 | 16.547159958962 | | 2017 | 16.529371753881 | | 2016 | 17.04907324715 | | 2015 | 16.863979442858 | | 2014 | 15.92938748746 | | 2013 | 17.741292000761 | | 2012 | 19.625100872621 | | 2011 | 20.545458986975 | | 2010 | 19.967616672683 | | 2009 | 17.33589682734 | | 2008 | 17.730924034091 | | 2007 | 23.954293125558 | | 2006 | 29.068942899269 | | 2005 | 30.106494615218 | | 2004 | 29.384836816314 | | 2003 | 26.258268878035 | | 2002 | 25.516716576412 | | 2001 | 28.925218786438 | | 2000 | 25.891349184655 | | 1999 | 14.899579345077 | | 1998 | 17.337435974722 | | 1997 | 5.5723943934034 | | 1996 | 16.132508522217 | | 1995 | 16.357511014713 | | 1994 | 3.1098432210837 | | 1993 | 11.208208122817 | | 1992 | -11.993580885562 | | 1991 | -10.951988960221 | | 1990 | 18.777757922362 | | 1989 | 26.82826806741 | | 1988 | 23.725266896975 | | 1987 | 26.060755588155 | | 1986 | 28.242566577327 | | 1985 | 28.410000436207 | | 1984 | 27.811226308299 |
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