| Year | Value | | 2022 | 7.6969543717686 | | 2021 | 5.5456543077348 | | 2020 | 5.6910747474621 | | 2019 | 5.5919963993063 | | 2018 | 5.5436213948647 | | 2017 | 5.7020701571725 | | 2016 | 5.5135257269299 | | 2015 | 6.1942802298499 | | 2014 | 6.9916388922086 | | 2013 | 7.5304064091317 | | 2012 | 6.2175042206926 | | 2011 | 11.39516515524 | | 2010 | 8.1266763916992 | | 2009 | 5.4234723617468 | | 2008 | 8.9019448946515 | | 2007 | 9.1069849690538 | | 2006 | 6.7652611705476 | | 2005 | 7.0466181622204 | | 2004 | 7.5875363850469 | | 2003 | 5.6687077344148 | | 2002 | 3.3325649327191 | | 2001 | 2.0071737421384 | | 2000 | 2.2082562093753 | | 1999 | 6.1066958984196 | | 1998 | 8.4022379561323 | | 1997 | 5.305601056688 | | 1996 | 2.3771290295322 | | 1995 | 10.297811794721 | | 1994 | 5.313740079365 | | 1993 | 3.014818599898 | | 1992 | 3.6340769179848 | | 1991 | 6.3573641227824 | | 1990 | 6.1267184698146 | | 1989 | 6.0454797559624 | | 1988 | 7.4127659574467 | | 1987 | 9.8746960912663 |
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