| Year | Value | | 2022 | 5.5181292857696 | | 2021 | 2.5447921362096 | | 2020 | 2.4062921438701 | | 2019 | 1.9332133581052 | | 2018 | 2.8287157847128 | | 2017 | 2.8829116498121 | | 2016 | 1.7571115890829 | | 2015 | 1.0864862994041 | | 2014 | 3.3704069737162 | | 2013 | 2.9275835157568 | | 2012 | 3.026963911248 | | 2011 | 5.159004976844 | | 2010 | 3.611962344303 | | 2009 | 3.675033055403 | | 2008 | 10.336826030069 | | 2007 | 4.8102370434291 | | 2006 | 4.6374743601176 | | 2005 | 4.5403691963453 | | 2004 | 4.5342137412969 | | 2003 | 3.2346481729393 | | 2002 | 2.3431699096098 | | 2001 | 4.2726702446511 | | 2000 | 2.1585181061942 | | 1999 | 4.2907285172381 | | 1998 | 8.6148315364729 | | 1997 | 5.6080284338985 | | 1996 | 7.3134825933035 | | 1995 | 5.8181818181819 | | 1994 | 7.6582430201954 | | 1993 | 5.736905833574 | | 1992 | 7.5235171702275 | | 1991 | 6.9656812776075 | | 1990 | 8.1913713204097 | | 1989 | 6.4625617114644 | | 1988 | 9.3421268854721 | | 1987 | 5.1748812732386 |
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