| Year | Value | | 2022 | 26.238582175851 | | 2021 | 25.831893247405 | | 2020 | 25.448592180658 | | 2019 | 25.440796226758 | | 2018 | 25.626599794707 | | 2017 | 25.983914348453 | | 2016 | 26.482855398048 | | 2015 | 26.831074672245 | | 2014 | 26.894741629095 | | 2013 | 26.732325922846 | | 2012 | 26.736223899796 | | 2011 | 26.686849525096 | | 2010 | 27.784779699336 | | 2009 | 27.741901952887 | | 2008 | 28.006964385484 | | 2007 | 28.557878461079 | | 2006 | 29.859802762366 | | 2005 | 30.962930239206 | | 2004 | 31.016202590855 | | 2003 | 30.367839091511 | | 2002 | 31.176019645804 | | 2001 | 30.925249795356 | | 2000 | 30.957732936606 | | 1999 | 31.54632745605 | | 1998 | 31.754219560048 | | 1997 | 31.56971531775 | | 1996 | 31.851668983797 | | 1995 | 32.033574574796 | | 1994 | 32.099840182945 | | 1993 | 31.808791237348 | | 1992 | 31.851668983797 | | 1991 | 32.049166482596 | | 1990 | 32.024479295246 | | 1989 | 32.327222171693 | | 1988 | 32.205085560594 | | 1987 | 32.436365526292 | | 1986 | 31.908839312397 | | 1985 | 31.956914361446 | | 1984 | 31.836077075998 | | 1983 | 30.788820602107 | | 1982 | 31.442381404051 | | 1981 | 33.110715538635 | | 1980 | 32.943102529787 | | 1979 | 33.494016605382 | | 1978 | 32.64295830464 | | 1977 | 32.564998765641 | | 1976 | 32.303834309993 | | 1975 | 32.363603289893 | | 1974 | 32.692332679339 |
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