| Year | Value | | 2023 | 1.439 | | 2022 | 1.412 | | 2021 | 1.427 | | 2020 | 1.489 | | 2019 | 1.54 | | 2018 | 1.607 | | 2017 | 1.585 | | 2016 | 1.6 | | 2015 | 1.685 | | 2014 | 1.655 | | 2013 | 1.678 | | 2012 | 1.668 | | 2011 | 1.734 | | 2010 | 1.673 | | 2009 | 1.678 | | 2008 | 1.576 | | 2007 | 1.436 | | 2006 | 1.402 | | 2005 | 1.486 | | 2004 | 1.546 | | 2003 | 1.627 | | 2002 | 1.645 | | 2001 | 1.595 | | 2000 | 1.608 | | 1999 | 1.64 | | 1998 | 1.617 | | 1997 | 1.59 | | 1996 | 1.457 | | 1995 | 1.481 | | 1994 | 1.467 | | 1993 | 1.489 | | 1992 | 1.562 | | 1991 | 1.663 | | 1990 | 1.806 | | 1989 | 1.832 | | 1988 | 1.853 | | 1987 | 1.8 | | 1986 | 1.779 | | 1985 | 1.772 | | 1984 | 1.774 | | 1983 | 1.801 | | 1982 | 1.776 | | 1981 | 1.591 | | 1980 | 1.637 | | 1979 | 1.771 | | 1978 | 1.884 | | 1977 | 2.209 | | 1976 | 2.506 | | 1975 | 2.824 |
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