| Year | Value | | 2022 | 50.368988396241 | | 2021 | 50.36715910546 | | 2020 | 50.363727224141 | | 2019 | 50.364187896889 | | 2018 | 50.371227899441 | | 2017 | 50.380598344662 | | 2016 | 50.387109977898 | | 2015 | 50.386757249138 | | 2014 | 50.384489086627 | | 2013 | 50.381506488463 | | 2012 | 50.378856922975 | | 2011 | 50.378386605979 | | 2010 | 50.376960652339 | | 2009 | 50.373806386773 | | 2008 | 50.370867814601 | | 2007 | 50.368432966539 | | 2006 | 50.36734656396 | | 2005 | 50.363161487282 | | 2004 | 50.354183546151 | | 2003 | 50.345044539694 | | 2002 | 50.335591338995 | | 2001 | 50.32478144455 | | 2000 | 50.314253072229 | | 1999 | 50.30552021196 | | 1998 | 50.299331681508 | | 1997 | 50.292562998804 | | 1996 | 50.28429098101 | | 1995 | 50.276711968457 | | 1994 | 50.270831629802 | | 1993 | 50.267952319646 | | 1992 | 50.267564252896 | | 1991 | 50.265732533202 | | 1990 | 50.260438760243 | | 1989 | 50.254828782829 | | 1988 | 50.252194758959 | | 1987 | 50.254468258094 | | 1986 | 50.254512724517 | | 1985 | 50.246308815651 | | 1984 | 50.228980524637 | | 1983 | 50.206867391201 | | 1982 | 50.189067744589 | | 1981 | 50.176681420838 | | 1980 | 50.165346856117 | | 1979 | 50.15311388175 | | 1978 | 50.139301050805 | | 1977 | 50.124801034398 | | 1976 | 50.112768604599 | | 1975 | 50.100836211671 | | 1974 | 50.089704292371 | | 1973 | 50.073458634698 | | 1972 | 50.046723928654 | | 1971 | 50.022895767768 | | 1970 | 50.006532266597 | | 1969 | 49.988396175388 | | 1968 | 49.967950278049 | | 1967 | 49.945884034214 | | 1966 | 49.920467374378 | | 1965 | 49.895774689225 | | 1964 | 49.86879388504 | | 1963 | 49.833156307886 | | 1962 | 49.796102341235 | | 1961 | 49.763933152383 | | 1960 | 49.733929268886 |
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