| Year | Value | | 2023 | 11.2 | | 2022 | 11.3 | | 2021 | 11.3 | | 2020 | 11.3 | | 2019 | 11.5 | | 2018 | 11.6 | | 2017 | 11.9 | | 2016 | 12.1 | | 2015 | 11.8 | | 2014 | 12.1 | | 2013 | 12 | | 2012 | 12 | | 2011 | 11.6 | | 2010 | 12 | | 2009 | 14 | | 2008 | 13.4 | | 2007 | 12.7 | | 2006 | 12.3 | | 2005 | 12 | | 2004 | 12.8 | | 2003 | 13.6 | | 2002 | 13.9 | | 2001 | 14.6 | | 2000 | 15.2 | | 1999 | 14.6 | | 1998 | 15.1 | | 1997 | 14.4 | | 1996 | 15 | | 1995 | 15.4 | | 1994 | 15.009 | | 1993 | 15.24 | | 1992 | 15.788 | | 1991 | 15.906 | | 1990 | 15.526 | | 1989 | 15.664 | | 1988 | 16.433 | | 1987 | 17.157 | | 1986 | 17.493 | | 1985 | 17.93 | | 1984 | 18.073 | | 1983 | 18.331 | | 1982 | 18.266 | | 1981 | 18.151 | | 1980 | 18.32 | | 1979 | 18.216 | | 1978 | 18.569 | | 1977 | 19.069 | | 1976 | 19.353 | | 1975 | 19.53 |
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