| Year | Value | | 2021 | 5.078 | | 2020 | 5.177 | | 2019 | 5.271 | | 2018 | 5.38 | | 2017 | 5.481 | | 2016 | 5.587 | | 2015 | 5.695 | | 2014 | 5.794 | | 2013 | 5.889 | | 2012 | 5.991 | | 2011 | 6.108 | | 2010 | 6.26 | | 2009 | 6.409 | | 2008 | 6.515 | | 2007 | 6.588 | | 2006 | 6.659 | | 2005 | 6.708 | | 2004 | 6.746 | | 2003 | 6.785 | | 2002 | 6.819 | | 2001 | 6.849 | | 2000 | 6.872 | | 1999 | 6.897 | | 1998 | 6.958 | | 1997 | 7.04 | | 1996 | 7.123 | | 1995 | 7.184 | | 1994 | 7.23 | | 1993 | 7.272 | | 1992 | 7.301 | | 1991 | 7.336 | | 1990 | 7.365 | | 1989 | 7.387 | | 1988 | 7.383 | | 1987 | 7.38 | | 1986 | 7.32 | | 1985 | 7.294 | | 1984 | 7.295 | | 1983 | 7.298 | | 1982 | 7.314 | | 1981 | 7.329 | | 1980 | 7.353 | | 1979 | 7.307 | | 1978 | 7.259 | | 1977 | 7.245 | | 1976 | 7.248 | | 1975 | 7.244 | | 1974 | 7.222 | | 1973 | 7.215 | | 1972 | 7.21 | | 1971 | 7.213 | | 1970 | 7.214 | | 1969 | 7.215 | | 1968 | 7.18 | | 1967 | 7.156 | | 1966 | 7.135 | | 1965 | 7.113 | | 1964 | 7.09 | | 1963 | 7.067 | | 1962 | 7.038 | | 1961 | 7.023 | | 1960 | 7.003 |
|