| Year | Value | | 2023 | 8.8 | | 2022 | 9 | | 2021 | 9.7 | | 2020 | 9.5 | | 2019 | 9.9 | | 2018 | 10.1 | | 2017 | 10.3 | | 2016 | 10.6 | | 2015 | 10.7 | | 2014 | 10.9 | | 2013 | 10.9 | | 2012 | 11 | | 2011 | 11 | | 2010 | 11.2 | | 2009 | 11.4 | | 2008 | 11.5 | | 2007 | 11.3 | | 2006 | 11 | | 2005 | 10.7 | | 2004 | 10.6 | | 2003 | 10.6 | | 2002 | 10.5 | | 2001 | 10.8 | | 2000 | 10.7 | | 1999 | 11.1 | | 1998 | 11.4 | | 1997 | 11.7 | | 1996 | 12.4 | | 1995 | 12.9 | | 1994 | 13.3 | | 1993 | 13.6 | | 1992 | 14.1 | | 1991 | 14.4 | | 1990 | 15 | | 1989 | 14.9 | | 1988 | 14.5 | | 1987 | 14.4 | | 1986 | 14.5 | | 1985 | 14.9 | | 1984 | 15 | | 1983 | 15 | | 1982 | 15.1 | | 1981 | 15.3 | | 1980 | 15.4 | | 1979 | 15.4 | | 1978 | 15.2 | | 1977 | 15.5 | | 1976 | 15.6 |
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