| Year | Value | | 2024 | 28.719406722755 | | 2023 | 28.483740964499 | | 2022 | 28.152935827869 | | 2021 | 28.177693498573 | | 2020 | 27.890443522382 | | 2019 | 26.710806599188 | | 2018 | 25.693245765701 | | 2017 | 25.040334960278 | | 2016 | 24.619523484903 | | 2015 | 24.723914754071 | | 2014 | 25.371387048075 | | 2013 | 25.909789283991 | | 2012 | 25.718610866559 | | 2011 | 25.138463237527 | | 2010 | 24.540596959214 | | 2009 | 23.940102771035 | | 2008 | 23.817993493761 | | 2007 | 25.180891862038 | | 2006 | 28.500942095801 | | 2005 | 31.672700046522 | | 2004 | 31.910186168184 | | 2003 | 31.316185969313 | | 2002 | 31.435521403815 | | 2001 | 31.561455913265 | | 2000 | 31.699765422356 | | 1999 | 31.855510204082 | | 1998 | 32.034946713497 | | 1997 | 32.216471904683 | | 1996 | 32.235137112278 | | 1995 | 32.091319513305 | | 1994 | 31.936691626957 | | 1993 | 31.771727644961 | | 1992 | 31.594479387129 | | 1991 | 31.403621397729 | | 1990 | 31.198202537167 | | 1989 | 30.974168614691 | | 1988 | 30.729335100239 | | 1987 | 30.460496818688 | | 1986 | 30.215834863057 | | 1985 | 30.2172487293 | | 1984 | 30.438257090129 | | 1983 | 30.670682328963 | | 1982 | 30.927626574051 | | 1981 | 31.222406933014 | | 1980 | 31.566834473185 | | 1979 | 31.975103413616 | | 1978 | 32.457290318312 | | 1977 | 33.022762379929 | | 1976 | 33.684603278626 |
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