| Year | Value | | 2022 | 5.4087437547907 | | 2021 | 2.410192998372 | | 2020 | 1.2807336187659 | | 2019 | 1.7039786323628 | | 2018 | 2.3126034330156 | | 2017 | 1.8003422246862 | | 2016 | 1.2769909449732 | | 2015 | 0.79527963057984 | | 2014 | 2.5108763325371 | | 2013 | 2.582687661418 | | 2012 | 2.9746077981447 | | 2011 | 5.0214601462073 | | 2010 | 3.0573056396244 | | 2009 | 1.6217301400345 | | 2008 | 8.3420863492096 | | 2007 | 3.764002591816 | | 2006 | 3.9361625992987 | | 2005 | 3.0370233261701 | | 2004 | 3.2375139400332 | | 2003 | 2.2891566265061 | | 2002 | 1.8857200024036 | | 2001 | 3.5775545209507 | | 2000 | 1.5919691746097 | | 1999 | 1.9838242723419 | | 1998 | 5.4910118038094 | | 1997 | 3.7276576018363 | | 1996 | 5.5248098601514 | | 1995 | 4.6277665995976 | | 1994 | 5.6499089394551 | | 1993 | 4.887555452635 | | 1992 | 6.2132814635518 | | 1991 | 6.4973455598455 | | 1990 | 7.8191914473055 | | 1989 | 6.4155396349497 | | 1988 | 7.8873239436618 | | 1987 | 4.685987787925 |
|