| Year | Value | | 2023 | 6.2 | | 2022 | 5.9 | | 2021 | 6.7 | | 2020 | 6.1 | | 2019 | 5.3 | | 2018 | 5.2 | | 2017 | 5.3 | | 2016 | 5.3 | | 2015 | 5.2 | | 2014 | 5.1 | | 2013 | 4.9 | | 2012 | 5 | | 2011 | 5.1 | | 2010 | 5 | | 2009 | 5.1 | | 2008 | 5.973 | | 2007 | 5.905 | | 2006 | 5.875 | | 2005 | 5.832 | | 2004 | 5.794 | | 2003 | 5.774 | | 2002 | 5.766 | | 2001 | 5.791 | | 2000 | 5.822 | | 1999 | 6.184 | | 1998 | 5.954 | | 1997 | 6.046 | | 1996 | 6.119 | | 1995 | 6.236 | | 1994 | 6.307 | | 1993 | 6.406 | | 1992 | 6.52 | | 1991 | 6.626 | | 1990 | 6.807 | | 1989 | 7.038 | | 1988 | 7.245 | | 1987 | 7.489 | | 1986 | 7.74 | | 1985 | 8.01 | | 1984 | 8.306 | | 1983 | 8.627 | | 1982 | 8.972 | | 1981 | 9.352 | | 1980 | 9.655 | | 1979 | 10 | | 1978 | 10.318 | | 1977 | 10.758 | | 1976 | 11.214 | | 1975 | 11.565 |
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