Year | Value | 2021 | 16.831727537531 | 2020 | 17.082679811786 | 2019 | 15.458131301815 | 2018 | 17.593930539996 | 2017 | 18.863453282545 | 2016 | 18.420345059377 | 2015 | 16.798790051535 | 2014 | 17.031369034282 | 2013 | 18.211965045933 | 2012 | 18.030472776159 | 2011 | 17.485547837777 | 2010 | 17.318619762492 | 2009 | 18.013892000896 | 2008 | 18.041676002689 | 2007 | 18.070804391665 | 2006 | 18.068563746359 | 2005 | 18.198521174098 | 2004 | 18.395697961013 | 2003 | 20.289043244454 | 2002 | 18.825901859736 | 2001 | 19.018597356039 | 2000 | 19.64373739637 | 1999 | 19.758010306968 | 1998 | 20.239749047726 | 1997 | 20.120994846516 | 1996 | 20.380909701994 | 1995 | 19.988796773471 | 1994 | 20.443647770558 | 1993 | 20.163567107327 | 1992 | 20.017925162447 | 1991 | 19.531705131078 | 1990 | 19.509298678019 | 1989 | 18.949137351557 | 1988 | 18.315034730002 | 1987 | 18.297109567555 | 1986 | 17.828814698633 | 1985 | 17.651803719471 | 1984 | 17.553215326014 | 1983 | 17.52856822765 | 1982 | 17.403092090522 | 1981 | 17.230562401972 | 1980 | 16.872059153036 | 1979 | 16.813802375084 | 1978 | 17.158861752185 | 1977 | 16.692807528568 | 1976 | 16.390320412279 | 1975 | 16.34102621555 | 1974 | 16.132646202106 | 1973 | 16.121442975577 | 1972 | 15.976609759376 | 1971 | 15.916117757763 | 1970 | 15.853385311646 | 1969 | 15.683111529327 | 1968 | 15.683111529327 | 1967 | 15.459067078909 | 1966 | 15.459067078909 | 1965 | 15.235022628489 | 1964 | 15.010978178071 | 1963 | 14.898955952861 | 1962 | 14.76452928261 | 1961 | 14.76452928261 |
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