| Year | Value | | 2021 | 61.666666666667 | | 2020 | 61.666666666667 | | 2019 | 61.666666666667 | | 2018 | 60.693641618497 | | 2017 | 60.693641618497 | | 2016 | 59.849096501346 | | 2015 | 59.945244956772 | | 2014 | 60.362238031148 | | 2013 | 60.854922279793 | | 2012 | 60.062447842924 | | 2011 | 59.962756052142 | | 2010 | 60.702743329575 | | 2009 | 61.19691845171 | | 2008 | 61.696730552424 | | 2007 | 62.194663660278 | | 2006 | 62.294465846096 | | 2005 | 62.354378053363 | | 2004 | 61.347617705103 | | 2003 | 62.387218045113 | | 2002 | 62.543472130839 | | 2001 | 61.973929236499 | | 2000 | 61.024208566108 | | 1999 | 62.011173184358 | | 1998 | 62.141527001862 | | 1997 | 62.262569832402 | | 1996 | 61.59217877095 | | 1995 | 62.234636871508 | | 1994 | 62.253258845438 | | 1993 | 62.662942271881 | | 1992 | 63.081936685289 | | 1991 | 62.895716945996 | | 1990 | 62.765363128492 | | 1989 | 63.054003724395 | | 1988 | 63.202979515829 | | 1987 | 62.625698324022 | | 1986 | 60.065176908752 | | 1985 | 57.932960893855 | | 1984 | 57.188081936685 | | 1983 | 57.532588454376 | | 1982 | 56.638733705773 | | 1981 | 55.288640595903 | | 1980 | 55.279329608938 | | 1979 | 54.581005586592 | | 1978 | 55.381750465549 | | 1977 | 56.219739292365 | | 1976 | 55.837988826816 | | 1975 | 54.096834264432 | | 1974 | 52.430167597765 | | 1973 | 54.478584729981 | | 1972 | 51.964618249535 | | 1971 | 47.234636871508 | | 1970 | 46.759776536313 | | 1969 | 44.180633147114 | | 1968 | 44.01303538175 | | 1967 | 43.026070763501 | | 1966 | 41.992551210428 | | 1965 | 42.346368715084 | | 1964 | 44.27374301676 | | 1963 | 37.597765363129 | | 1962 | 35.428305400372 | | 1961 | 33.054003724395 |
|