| Year | Value | | 2022 | 22.481438732683 | | 2021 | 20.202918833377 | | 2020 | 24.81817336064 | | 2019 | 31.755187209583 | | 2018 | 33.807050717959 | | 2017 | 35.488631675215 | | 2016 | 35.054772420268 | | 2015 | 35.634833372454 | | 2014 | 37.34744797501 | | 2013 | 36.354401712467 | | 2012 | 35.132849346384 | | 2011 | 36.875071084182 | | 2010 | 39.751491085044 | | 2009 | 37.948691712028 | | 2008 | 36.022274740875 | | 2007 | 36.754422362675 | | 2006 | 37.228201627585 | | 2005 | 37.771455095585 | | 2004 | 36.229230916294 | | 2003 | 35.609955698205 | | 2002 | 35.26884428528 | | 2001 | 34.62368334017 | | 2000 | 33.391674129253 | | 1999 | 35.967801081711 | | 1998 | 29.619158047975 | | 1997 | 23.912233786787 | | 1996 | 22.406747144851 | | 1995 | 21.935132515577 | | 1994 | 22.42090706199 | | 1993 | 20.351273437458 | | 1992 | 20.710067427688 | | 1991 | 20.994025916504 | | 1990 | 22.439732436883 | | 1989 | 22.174050812692 | | 1988 | 22.62683359253 | | 1987 | 21.808914189587 | | 1986 | 18.491565172527 | | 1985 | 18.208976111179 | | 1984 | 23.769969829639 | | 1983 | 30.621159992346 | | 1982 | 28.090316598668 | | 1981 | 30.494346621442 |
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