| Year | Value | | 2021 | 13.999192276294 | | 2020 | 12.276716036242 | | 2019 | 13.374207610723 | | 2018 | 13.182148685143 | | 2017 | 13.244102554457 | | 2016 | 13.510854335456 | | 2015 | 13.253994216314 | | 2014 | 13.045588962406 | | 2013 | 13.41682474966 | | 2012 | 13.105296065848 | | 2011 | 13.208986865127 | | 2010 | 12.952542058471 | | 2009 | 13.183071988107 | | 2008 | 15.232509906224 | | 2007 | 14.856470156166 | | 2006 | 13.841663738386 | | 2005 | 13.635396985214 | | 2004 | 13.358927500176 | | 2003 | 13.584951609795 | | 2002 | 13.368108901546 | | 2001 | 13.361033803688 | | 2000 | 12.839712833354 | | 1999 | 13.133758163939 | | 1998 | 12.823864932676 | | 1997 | 12.613164915421 | | 1996 | 12.642815237021 | | 1995 | 12.245037959189 | | 1994 | 11.580786930116 | | 1993 | 12.035269313786 | | 1992 | 11.976548253817 | | 1991 | 10.586351507218 | | 1990 | 13.057334647419 | | 1989 | 13.704750784613 | | 1988 | 14.70223261208 | | 1987 | 14.434157817676 | | 1986 | 13.257398329868 | | 1985 | 13.273053024508 | | 1984 | 14.716807280986 | | 1983 | 15.218769816106 | | 1982 | 12.655748354281 | | 1981 | 12.381200780513 | | 1980 | 11.616813966902 | | 1979 | 11.883970601474 | | 1978 | 12.65156227999 | | 1977 | 12.305028204815 | | 1976 | 12.188279904815 | | 1975 | 12.437070876052 | | 1974 | 13.620163999033 | | 1973 | 12.497047818458 | | 1972 | 11.441369069354 |
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