| Year | Value | | 2022 | 7.9309292543009 | | 2021 | 2.4108019840317 | | 2020 | 1.8338835031519 | | 2019 | 1.8584121912698 | | 2018 | 2.647070732656 | | 2017 | 2.3802359905313 | | 2016 | 1.5031205586107 | | 2015 | 1.5486915822249 | | 2014 | 2.1200017571813 | | 2013 | 3.2609945685887 | | 2012 | 4.0393235613905 | | 2011 | 4.800635941694 | | 2010 | 2.9522040669278 | | 2009 | 2.6558020584776 | | 2008 | 9.0794396457087 | | 2007 | 5.1038343762531 | | 2006 | 3.7097847980387 | | 2005 | 3.3291992280286 | | 2004 | 3.2375139400332 | | 2003 | 2.9374312405021 | | 2002 | 3.4165819479591 | | 2001 | 3.3823770921475 | | 2000 | 4.0800735860541 | | 1999 | 3.2318326090647 | | 1998 | 2.9513888888889 | | 1997 | 4.8344898191332 | | 1996 | 4.9045346062053 | | 1995 | 6.9809744183056 | | 1994 | 4.6985893105498 | | 1993 | 5.788395904437 | | 1992 | 4.2956155335387 | | 1991 | 6.9048963274276 | | 1990 | 8.8685801053763 | | 1989 | 6.9533482955214 | | 1988 | 5.5294353641351 | | 1987 | 3.3189102564105 | | 1986 | 8.0037741996751 | | 1985 | 5.2138560453072 | | 1984 | 6.5148288610274 | | 1983 | 6.0526315789474 | | 1982 | 9.846772167797 | | 1981 | 10.744693608398 | | 1980 | 13.571540510809 |
|