| Year | Value | | 2023 | 1.463 | | 2022 | 1.477 | | 2021 | 1.501 | | 2020 | 1.552 | | 2019 | 1.617 | | 2018 | 1.677 | | 2017 | 1.704 | | 2016 | 1.713 | | 2015 | 1.718 | | 2014 | 1.729 | | 2013 | 1.752 | | 2012 | 1.805 | | 2011 | 1.859 | | 2010 | 1.907 | | 2009 | 1.959 | | 2008 | 2.005 | | 2007 | 2.049 | | 2006 | 2.078 | | 2005 | 2.106 | | 2004 | 2.136 | | 2003 | 2.176 | | 2002 | 2.242 | | 2001 | 2.342 | | 2000 | 2.406 | | 1999 | 2.464 | | 1998 | 2.52 | | 1997 | 2.581 | | 1996 | 2.649 | | 1995 | 2.699 | | 1994 | 2.756 | | 1993 | 2.842 | | 1992 | 2.939 | | 1991 | 3.053 | | 1990 | 3.189 | | 1989 | 3.335 | | 1988 | 3.499 | | 1987 | 3.653 | | 1986 | 3.789 | | 1985 | 3.908 | | 1984 | 3.986 | | 1983 | 4.085 | | 1982 | 4.192 | | 1981 | 4.291 | | 1980 | 4.37 | | 1979 | 4.43 | | 1978 | 4.485 | | 1977 | 4.553 | | 1976 | 4.624 | | 1975 | 4.687 |
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