| Year | Value | | 2021 | 4.5644446224433 | | 2020 | 23.480333704131 | | 2019 | 28.336535314246 | | 2018 | 22.32057913543 | | 2017 | 21.671422187553 | | 2016 | 14.19605809827 | | 2015 | 9.7022724983094 | | 2014 | 15.573529465072 | | 2013 | 15.922598265164 | | 2012 | 20.973302782791 | | 2011 | 16.372288132933 | | 2010 | 11.298215194448 | | 2009 | 7.9173795485944 | | 2008 | 8.5451243514552 | | 2007 | 11.09487875705 | | 2006 | 15.339125975493 | | 2005 | 12.582555390369 | | 2004 | 16.947972270195 | | 2003 | 15.249694557479 | | 2002 | 19.299498263847 | | 2001 | 19.294346924158 | | 1994 | 13.649535230847 | | 1993 | 20.794718263143 | | 1992 | 20.658331619421 | | 1991 | 9.127236037661 | | 1990 | 26.239029536549 | | 1989 | 19.602683592002 | | 1988 | 11.40968731863 | | 1987 | 26.716130730156 | | 1986 | 23.468407176012 | | 1985 | 19.600566226703 | | 1984 | 24.68484092922 | | 1983 | 32.638063559111 | | 1982 | 39.850812315022 | | 1981 | 17.946066483683 |
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