| Year | Value | | 2023 | 1.619 | | 2022 | 1.629 | | 2021 | 1.638 | | 2020 | 1.653 | | 2019 | 1.705 | | 2018 | 1.744 | | 2017 | 1.74 | | 2016 | 1.727 | | 2015 | 1.777 | | 2014 | 1.775 | | 2013 | 1.761 | | 2012 | 1.766 | | 2011 | 1.777 | | 2010 | 1.786 | | 2009 | 1.799 | | 2008 | 1.815 | | 2007 | 1.854 | | 2006 | 1.896 | | 2005 | 1.942 | | 2004 | 1.973 | | 2003 | 2.006 | | 2002 | 2.081 | | 2001 | 2.162 | | 2000 | 2.247 | | 1999 | 2.335 | | 1998 | 2.418 | | 1997 | 2.468 | | 1996 | 2.511 | | 1995 | 2.567 | | 1994 | 2.603 | | 1993 | 2.66 | | 1992 | 2.71 | | 1991 | 2.805 | | 1990 | 2.896 | | 1989 | 3.007 | | 1988 | 3.112 | | 1987 | 3.232 | | 1986 | 3.361 | | 1985 | 3.48 | | 1984 | 3.6 | | 1983 | 3.723 | | 1982 | 3.838 | | 1981 | 3.943 | | 1980 | 4.037 | | 1979 | 4.12 | | 1978 | 4.195 | | 1977 | 4.267 | | 1976 | 4.339 | | 1975 | 4.415 |
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