| Year | Value | | 2023 | 1.823 | | 2022 | 1.861 | | 2021 | 1.909 | | 2020 | 1.967 | | 2019 | 2.035 | | 2018 | 2.111 | | 2017 | 2.128 | | 2016 | 2.122 | | 2015 | 2.209 | | 2014 | 2.274 | | 2013 | 2.362 | | 2012 | 2.509 | | 2011 | 2.605 | | 2010 | 2.615 | | 2009 | 2.678 | | 2008 | 2.688 | | 2007 | 2.704 | | 2006 | 2.753 | | 2005 | 2.8 | | 2004 | 2.853 | | 2003 | 2.911 | | 2002 | 2.972 | | 2001 | 3.037 | | 2000 | 3.102 | | 1999 | 3.166 | | 1998 | 3.228 | | 1997 | 3.288 | | 1996 | 3.351 | | 1995 | 3.381 | | 1994 | 3.42 | | 1993 | 3.491 | | 1992 | 3.575 | | 1991 | 3.633 | | 1990 | 3.68 | | 1989 | 3.736 | | 1988 | 3.787 | | 1987 | 3.87 | | 1986 | 4 | | 1985 | 4.145 | | 1984 | 4.305 | | 1983 | 4.395 | | 1982 | 4.501 | | 1981 | 4.612 | | 1980 | 4.73 | | 1979 | 4.855 | | 1978 | 4.989 | | 1977 | 5.13 | | 1976 | 5.276 | | 1975 | 5.426 |
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