| Year | Value | | 2021 | 5.343 | | 2020 | 5.305 | | 2019 | 5.5 | | 2018 | 5.297 | | 2017 | 5.29 | | 2016 | 5.29 | | 2015 | 5.281 | | 2014 | 5.285 | | 2013 | 5.299 | | 2012 | 5.372 | | 2011 | 5.308 | | 2010 | 5.275 | | 2009 | 6.066 | | 2008 | 5.238 | | 2007 | 5.221 | | 2006 | 5.169 | | 2005 | 5.171 | | 2004 | 5.088 | | 2003 | 5.057 | | 2002 | 5.035 | | 2001 | 5.08 | | 2000 | 5.138 | | 1999 | 5.206 | | 1998 | 5.276 | | 1997 | 5.353 | | 1996 | 5.429 | | 1995 | 5.505 | | 1994 | 5.594 | | 1993 | 5.662 | | 1992 | 5.741 | | 1991 | 5.9 | | 1990 | 5.95 | | 1989 | 5.987 | | 1988 | 6.068 | | 1987 | 6.158 | | 1986 | 6.198 | | 1985 | 6.229 | | 1984 | 6.271 | | 1983 | 6.324 | | 1982 | 6.381 | | 1981 | 6.494 | | 1980 | 6.631 | | 1979 | 6.779 | | 1978 | 6.942 | | 1977 | 7.09 | | 1976 | 7.212 | | 1975 | 7.351 | | 1974 | 7.532 | | 1973 | 7.765 | | 1972 | 8.018 | | 1971 | 8.283 | | 1970 | 8.562 | | 1969 | 8.85 | | 1968 | 9.15 | | 1967 | 9.463 | | 1966 | 9.915 | | 1965 | 10.275 | | 1964 | 12.897 | | 1963 | 11.354 | | 1962 | 12.008 | | 1961 | 12.307 | | 1960 | 12.747 |
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