| Year | Value | | 2024 | 39.89337168207 | | 2023 | 39.291937079579 | | 2022 | 38.705520716904 | | 2021 | 37.869205885937 | | 2020 | 37.00976715608 | | 2019 | 36.183763826636 | | 2018 | 35.40855388079 | | 2017 | 34.701495301727 | | 2016 | 34.079946072631 | | 2015 | 33.561264176687 | | 2014 | 33.16280759708 | | 2013 | 32.901934316994 | | 2012 | 32.793175212833 | | 2011 | 32.756729577697 | | 2010 | 32.728452581005 | | 2009 | 32.707428857318 | | 2008 | 32.691278620618 | | 2007 | 32.67762208489 | | 2006 | 32.664079464119 | | 2005 | 32.64827097229 | | 2004 | 32.627816823387 | | 2003 | 32.600337231395 | | 2002 | 32.563188416693 | | 2001 | 32.499927838284 | | 2000 | 32.399156885331 | | 1999 | 32.263782940747 | | 1998 | 32.09649244887 | | 1997 | 31.899971854041 | | 1996 | 31.676907600602 | | 1995 | 31.429986132892 | | 1994 | 31.161893895252 | | 1993 | 30.875317332021 | | 1992 | 30.567424698803 | | 1991 | 30.15558500778 | | 1990 | 29.592041213748 | | 1989 | 28.91158749248 | | 1988 | 28.145002143217 | | 1987 | 27.323063465201 | | 1986 | 26.476549757673 | | 1985 | 25.636239319876 | | 1984 | 24.83291045105 | | 1983 | 24.097341450438 | | 1982 | 23.457692494236 | | 1981 | 22.882691078088 | | 1980 | 22.331270861382 | | 1979 | 21.8015532364 | | 1978 | 21.289912223785 | | 1977 | 20.792721844179 | | 1976 | 20.306356118225 | | 1975 | 19.827189066566 |
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