| Year | Value | | 2021 | 21.707883389231 | | 2020 | 21.742435410754 | | 2019 | 21.581192643644 | | 2018 | 21.535123281613 | | 2017 | 21.489053919581 | | 2016 | 21.44298455755 | | 2015 | 21.350845833487 | | 2014 | 21.350845833487 | | 2013 | 20.417365385324 | | 2012 | 19.728541167803 | | 2011 | 19.68594584638 | | 2010 | 18.926137410205 | | 2009 | 18.074230981765 | | 2008 | 18.725564167638 | | 2007 | 20.897638155338 | | 2006 | 20.554679104648 | | 2005 | 20.211720053959 | | 2004 | 19.86876100327 | | 2003 | 19.86876100327 | | 2002 | 19.86876100327 | | 2001 | 19.411482269017 | | 2000 | 19.43434620573 | | 1999 | 19.43434620573 | | 1998 | 19.777305256419 | | 1997 | 19.800169193132 | | 1996 | 20.806182408487 | | 1995 | 20.806182408487 | | 1994 | 21.377780826303 | | 1993 | 22.566705535359 | | 1992 | 21.835059560555 | | 1991 | 22.017971054256 | | 1990 | 21.103413585751 | | 1989 | 21.103413585751 | | 1988 | 21.78933168713 | | 1987 | 21.675012003567 | | 1986 | 21.675012003567 | | 1985 | 21.640716098498 | | 1984 | 21.606420193429 | | 1983 | 21.606420193429 | | 1982 | 21.606420193429 | | 1981 | 21.581269863045 | | 1980 | 21.581269863045 | | 1979 | 21.581269863045 | | 1978 | 21.466950179482 | | 1977 | 21.229165237671 | | 1976 | 21.229165237671 | | 1975 | 21.229165237671 | | 1974 | 21.217733269314 | | 1973 | 20.966229965475 | | 1972 | 20.943366028763 | | 1971 | 20.57525664769 | | 1970 | 20.561538285662 | | 1969 | 20.554679104648 | | 1968 | 20.543247136292 | | 1967 | 20.531815167936 | | 1966 | 20.406063516016 | | 1965 | 20.39463154766 | | 1964 | 20.268879895741 | | 1963 | 20.371767610947 | | 1962 | 20.246015959028 | | 1961 | 20.120264307108 |
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