| Year | Value | | 2022 | 33.568870011157 | | 2021 | 33.568870011157 | | 2020 | 33.220262091644 | | 2019 | 32.883791031338 | | 2018 | 32.77749907025 | | 2017 | 32.780050500108 | | 2016 | 33.01493472176 | | 2015 | 33.101058936366 | | 2014 | 33.35747421167 | | 2013 | 33.933460040322 | | 2012 | 34.12084793204 | | 2011 | 34.308755309362 | | 2010 | 34.300925835307 | | 2009 | 34.345945311124 | | 2008 | 34.915539548631 | | 2007 | 35.124977979604 | | 2006 | 35.140636927714 | | 2005 | 35.134764822173 | | 2004 | 35.06038481865 | | 2003 | 34.880306915383 | | 2002 | 34.803969543346 | | 2001 | 34.719802697254 | | 2000 | 34.596488480886 | | 1999 | 34.482961107088 | | 1998 | 34.428154788702 | | 1997 | 34.091487404334 | | 1996 | 33.713715281176 | | 1995 | 33.392706844918 | | 1994 | 33.427939478166 | | 1993 | 33.441641057762 | | 1992 | 33.741118440369 | | 1991 | 34.273522676114 | | 1990 | 34.242204779894 | | 1989 | 34.412495840592 | | 1988 | 34.700229012116 | | 1987 | 35.095617451898 | | 1986 | 34.860733230245 | | 1985 | 34.631721114134 | | 1984 | 34.060169508113 | | 1983 | 33.848773708626 | | 1982 | 33.664781068332 | | 1981 | 32.991446299595 | | 1980 | 32.325941004913 | | 1979 | 31.934467302159 | | 1978 | 31.529292019809 | | 1977 | 30.873573567696 | | 1976 | 29.68153614281 | | 1975 | 29.164790855174 | | 1974 | 28.624557145374 |
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