| Year | Value | | 2024 | 21.376524474644 | | 2023 | 21.937255037171 | | 2022 | 24.35491714342 | | 2021 | 23.303385198668 | | 2020 | 19.461889002372 | | 2019 | 20.261633986119 | | 2018 | 19.582931230402 | | 2017 | 19.845280989292 | | 2016 | 19.146454571981 | | 2015 | 19.569363631568 | | 2014 | 22.148493894754 | | 2013 | 21.597298402506 | | 2012 | 20.917951228194 | | 2011 | 20.857109411382 | | 2010 | 19.312530097616 | | 2009 | 18.520013474572 | | 2008 | 24.247409305481 | | 2007 | 24.404319557488 | | 2006 | 24.617277079787 | | 2005 | 24.451754139057 | | 2004 | 23.660504071832 | | 2003 | 21.943777150402 | | 2002 | 21.724737198022 | | 2001 | 22.635489053482 | | 2000 | 23.774628073122 | | 1999 | 21.095886781962 | | 1998 | 19.774214276071 | | 1997 | 20.196734015002 | | 1996 | 19.461567580578 | | 1995 | 18.949594812256 | | 1994 | 16.982990179113 | | 1993 | 14.883827947969 | | 1992 | 14.056917651052 | | 1991 | 15.142207882611 | | 1990 | 17.759890764973 | | 1989 | 20.138475622311 | | 1988 | 20.96099337985 | | 1987 | 20.166792347543 | | 1986 | 18.967020544479 | | 1985 | 20.534191947883 | | 1984 | 20.912067028604 | | 1983 | 20.052433363505 | | 1982 | 19.900019725663 | | 1981 | 22.4920879562 | | 1980 | 22.170476495759 | | 1979 | 22.051404291622 | | 1978 | 20.299221587014 | | 1977 | 20.598888909271 | | 1976 | 21.883763115668 |
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