| Year | Value | | 2023 | 1.74 | | 2022 | 1.8 | | 2021 | 1.75 | | 2020 | 1.75 | | 2019 | 1.8 | | 2018 | 1.8 | | 2017 | 1.8 | | 2016 | 1.8 | | 2015 | 1.7 | | 2014 | 1.75 | | 2013 | 1.73 | | 2012 | 1.72 | | 2011 | 1.65 | | 2010 | 1.7 | | 2009 | 1.98 | | 2008 | 1.89 | | 2007 | 1.8 | | 2006 | 1.73 | | 2005 | 1.69 | | 2004 | 1.734 | | 2003 | 1.804 | | 2002 | 1.813 | | 2001 | 1.855 | | 2000 | 1.903 | | 1999 | 1.884 | | 1998 | 1.893 | | 1997 | 1.886 | | 1996 | 1.922 | | 1995 | 1.958 | | 1994 | 1.936 | | 1993 | 1.955 | | 1992 | 2.013 | | 1991 | 2.015 | | 1990 | 1.945 | | 1989 | 1.938 | | 1988 | 2.006 | | 1987 | 2.07 | | 1986 | 2.091 | | 1985 | 2.129 | | 1984 | 2.136 | | 1983 | 2.166 | | 1982 | 2.186 | | 1981 | 2.193 | | 1980 | 2.217 | | 1979 | 2.197 | | 1978 | 2.247 | | 1977 | 2.307 | | 1976 | 2.352 | | 1975 | 2.381 |
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