| Year | Value | | 2021 | 5.459 | | 2020 | 5.352 | | 2019 | 5.318 | | 2018 | 5.241 | | 2017 | 5.183 | | 2016 | 5.13 | | 2015 | 5.125 | | 2014 | 5.077 | | 2013 | 5.019 | | 2012 | 4.928 | | 2011 | 4.876 | | 2010 | 4.829 | | 2009 | 4.723 | | 2008 | 4.628 | | 2007 | 4.563 | | 2006 | 4.478 | | 2005 | 4.35 | | 2004 | 4.233 | | 2003 | 4.146 | | 2002 | 4.061 | | 2001 | 3.99 | | 2000 | 3.878 | | 1999 | 3.789 | | 1998 | 3.795 | | 1997 | 3.668 | | 1996 | 3.589 | | 1995 | 3.512 | | 1994 | 3.408 | | 1993 | 3.296 | | 1992 | 3.213 | | 1991 | 3.088 | | 1990 | 3.009 | | 1979 | 1.6 | | 1978 | 1.5 | | 1977 | 1.5 | | 1976 | 1.5 | | 1975 | 1.5 | | 1974 | 1.4 | | 1973 | 1.4 | | 1972 | 1.4 | | 1971 | 1.4 | | 1970 | 1.4 | | 1969 | 1.4 | | 1968 | 1.4 | | 1967 | 1.4 | | 1966 | 1.4 | | 1965 | 1.4 | | 1964 | 1.4 | | 1963 | 1.4 | | 1962 | 1.4 | | 1961 | 1.4 | | 1960 | 1.2 |
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