| Year | Value | | 2023 | 5.059 | | 2022 | 5.004 | | 2021 | 5.648 | | 2020 | 5.886 | | 2019 | 5.004 | | 2018 | 5.162 | | 2017 | 5.115 | | 2016 | 5.15 | | 2015 | 5.136 | | 2014 | 5.154 | | 2013 | 5.165 | | 2012 | 5.129 | | 2011 | 5.182 | | 2010 | 5.224 | | 2009 | 5.247 | | 2008 | 5.27 | | 2007 | 5.349 | | 2006 | 5.351 | | 2005 | 5.425 | | 2004 | 5.533 | | 2003 | 5.657 | | 2002 | 5.836 | | 2001 | 6.102 | | 2000 | 6.35 | | 1999 | 6.657 | | 1998 | 6.972 | | 1997 | 7.276 | | 1996 | 7.575 | | 1995 | 8.463 | | 1994 | 8.142 | | 1993 | 8.34 | | 1992 | 8.536 | | 1991 | 8.728 | | 1990 | 8.918 | | 1989 | 9.185 | | 1988 | 9.43 | | 1987 | 9.67 | | 1986 | 9.903 | | 1985 | 10.109 | | 1984 | 10.385 | | 1983 | 10.463 | | 1982 | 10.632 | | 1981 | 10.782 | | 1980 | 10.949 | | 1979 | 11.214 | | 1978 | 11.474 | | 1977 | 11.76 | | 1976 | 12.133 | | 1975 | 12.376 |
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