| Year | Value | | 2021 | 11.915429146741 | | 2020 | 11.915429146741 | | 2019 | 11.915429146741 | | 2018 | 11.915429146741 | | 2017 | 11.711552982633 | | 2016 | 11.739239869116 | | 2015 | 12.252705763906 | | 2014 | 11.736722879436 | | 2013 | 11.286181726655 | | 2012 | 10.974075006292 | | 2011 | 10.375031462371 | | 2010 | 10.005033979361 | | 2009 | 9.7105461867606 | | 2008 | 9.4563302290461 | | 2007 | 9.161842436446 | | 2006 | 8.9353133652152 | | 2005 | 8.7087842939844 | | 2004 | 8.4822552227536 | | 2003 | 8.2557261515228 | | 2002 | 8.0795368738988 | | 2001 | 7.8152529574629 | | 2000 | 7.6013088346338 | | 1999 | 7.4251195570098 | | 1998 | 7.1734205889756 | | 1997 | 6.54417316889 | | 1996 | 6.54417316889 | | 1995 | 6.54417316889 | | 1994 | 6.1666247168387 | | 1993 | 5.7890762647873 | | 1992 | 5.7890762647873 | | 1991 | 5.411527812736 | | 1990 | 5.3108482255223 | | 1989 | 5.2101686383086 | | 1988 | 5.1094890510949 | | 1987 | 5.0088094638812 | | 1986 | 4.9081298766675 | | 1985 | 4.7822803926504 | | 1984 | 4.6816008054367 | | 1983 | 4.580921218223 | | 1982 | 4.4802416310093 | | 1981 | 4.382079033476 | | 1980 | 4.0775232821545 | | 1979 | 3.7754845205135 | | 1978 | 3.4986156556758 | | 1977 | 3.1965768940347 | | 1976 | 2.9197080291971 | | 1975 | 2.617669267556 | | 1974 | 2.3156305059149 | | 1973 | 2.0236597029952 | | 1972 | 2.0261766926756 | | 1971 | 2.0261766926756 | | 1970 | 2.0261766926756 | | 1969 | 2.0261766926756 | | 1968 | 2.0261766926756 | | 1967 | 2.0362446513969 | | 1966 | 2.0362446513969 | | 1965 | 1.9733199093884 | | 1964 | 1.9154291467405 | | 1963 | 1.8348854769695 | | 1962 | 1.7895796627234 | | 1961 | 1.7618927762396 |
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