| Year | Value | | 2021 | 13.115863848871 | | 2020 | 13.115863848871 | | 2019 | 13.115863848871 | | 2018 | 13.115863848871 | | 2017 | 13.115863848871 | | 2016 | 13.115863848871 | | 2015 | 13.115863848871 | | 2014 | 13.115863848871 | | 2013 | 13.115863848871 | | 2012 | 13.115863848871 | | 2011 | 13.115863848871 | | 2010 | 13.115863848871 | | 2009 | 12.614499375939 | | 2008 | 12.614499375939 | | 2007 | 12.614499375939 | | 2006 | 12.614499375939 | | 2005 | 12.614499375939 | | 2004 | 12.608152990205 | | 2003 | 12.608152990205 | | 2002 | 12.608152990205 | | 2001 | 12.608152990205 | | 2000 | 12.608152990205 | | 1999 | 12.608152990205 | | 1998 | 12.608152990205 | | 1997 | 12.608152990205 | | 1996 | 12.608152990205 | | 1995 | 12.608152990205 | | 1994 | 12.608152990205 | | 1993 | 12.608152990205 | | 1992 | 12.608152990205 | | 1991 | 12.586998371094 | | 1990 | 12.565843751983 | | 1989 | 12.565843751983 | | 1988 | 12.565843751983 | | 1987 | 12.544689132872 | | 1986 | 12.544689132872 | | 1985 | 12.50237989465 | | 1984 | 12.50237989465 | | 1983 | 12.50237989465 | | 1982 | 12.50237989465 | | 1981 | 12.50237989465 | | 1980 | 12.50237989465 | | 1979 | 12.476994351717 | | 1978 | 11.92908971674 | | 1977 | 11.98620718834 | | 1976 | 11.846586702206 | | 1975 | 11.740813606651 | | 1974 | 11.635040511096 | | 1973 | 11.52926741554 | | 1972 | 11.52926741554 | | 1971 | 11.52926741554 | | 1970 | 11.423494319985 | | 1969 | 11.423494319985 | | 1968 | 11.317721224429 | | 1967 | 11.211948128874 | | 1966 | 11.106175033319 | | 1965 | 11.000401937763 | | 1964 | 10.894628842208 | | 1963 | 10.788855746652 | | 1962 | 10.683082651097 | | 1961 | 10.577309555542 |
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