| Year | Value | | 2021 | 41.942231593043 | | 2020 | 42.332263616904 | | 2019 | 44.369995637721 | | 2018 | 41.948748469826 | | 2017 | 43.374329597793 | | 2016 | 43.325059685241 | | 2015 | 43.774812490278 | | 2014 | 44.508619698497 | | 2013 | 46.091208516221 | | 2012 | 46.425986919971 | | 2011 | 46.843952684652 | | 2010 | 48.45088902265 | | 2009 | 47.282884369568 | | 2008 | 48.888468067551 | | 2007 | 47.89021973637 | | 2006 | 48.028865337923 | | 2005 | 49.83125815811 | | 2004 | 50.328353363678 | | 2003 | 50.409511764586 | | 2002 | 51.647177378448 | | 2001 | 52.421563787121 | | 2000 | 52.878079792235 | | 1999 | 53.717996667913 | | 1998 | 52.646968821189 | | 1997 | 52.17435653327 | | 1996 | 52.187956886879 | | 1995 | 52.133555472442 | | 1994 | 53.38818809289 | | 1993 | 54.095406480568 | | 1992 | 54.326612491925 | | 1991 | 54.5850192105 | | 1990 | 57.257488694706 | | 1989 | 57.291489578729 | | 1988 | 57.254088606304 | | 1987 | 57.842303899901 | | 1986 | 57.94430655197 | | 1985 | 58.124511237292 | | 1984 | 58.631124409235 | | 1983 | 58.685525823671 | | 1982 | 59.623950222706 | | 1981 | 59.67495154874 | | 1980 | 59.712352521165 | | 1979 | 59.868756587671 | | 1978 | 59.804154908028 | | 1977 | 59.504947128625 | | 1976 | 59.60014960389 | | 1975 | 59.559348543062 | | 1974 | 59.508347217028 | | 1973 | 59.440345448982 | | 1972 | 59.528747747441 | | 1971 | 60.008160212165 | | 1970 | 68.613783958383 | | 1969 | 68.773588113291 | | 1968 | 69.208799428785 | | 1967 | 69.29040155044 | | 1966 | 69.3788038489 | | 1965 | 69.497806942981 | | 1964 | 69.572608887831 | | 1963 | 69.735813131141 | | 1962 | 70.218625684268 | | 1961 | 70.324028424739 |
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