| Year | Value | | 2022 | 49.731332988446 | | 2021 | 49.778318276581 | | 2020 | 49.840609211496 | | 2019 | 49.912045245888 | | 2018 | 50.001404099972 | | 2017 | 50.091261091124 | | 2016 | 50.180441800471 | | 2015 | 50.270994092541 | | 2014 | 50.349717590581 | | 2013 | 50.408113056 | | 2012 | 50.461660947354 | | 2011 | 50.517267924132 | | 2010 | 50.574158652927 | | 2009 | 50.606282298736 | | 2008 | 50.589741462085 | | 2007 | 50.553601719554 | | 2006 | 50.514788202007 | | 2005 | 50.433838135632 | | 2004 | 50.402054900712 | | 2003 | 50.465091505635 | | 2002 | 50.542320062153 | | 2001 | 50.625920960055 | | 2000 | 50.713876237378 | | 1999 | 50.805552740906 | | 1998 | 50.901972212299 | | 1997 | 50.997260352843 | | 1996 | 51.045637623355 | | 1995 | 51.050320228742 | | 1994 | 51.053957718781 | | 1993 | 51.056179304127 | | 1992 | 51.054428707949 | | 1991 | 51.051617873652 | | 1990 | 51.053231338569 | | 1989 | 51.047610247407 | | 1988 | 51.04794835786 | | 1987 | 51.074041733161 | | 1986 | 51.110278444935 | | 1985 | 51.148388874278 | | 1984 | 51.196453063698 | | 1983 | 51.267675096203 | | 1982 | 51.365099001757 | | 1981 | 51.486407327529 | | 1980 | 51.618113177074 | | 1979 | 51.754714284742 | | 1978 | 51.897949911195 | | 1977 | 52.037385785032 | | 1976 | 52.146467339148 | | 1975 | 52.219237048045 | | 1974 | 52.278275948616 | | 1973 | 52.335907642049 | | 1972 | 52.396869260172 | | 1971 | 52.460856873904 | | 1970 | 52.52653583464 | | 1969 | 52.575193783918 | | 1968 | 52.624028227448 | | 1967 | 52.701596806387 | | 1966 | 52.79160210563 | | 1965 | 52.884402392329 | | 1964 | 52.977849096291 | | 1963 | 53.047174863876 | | 1962 | 53.069281858259 | | 1961 | 53.068583465232 | | 1960 | 53.057757644394 |
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