| Year | Value | | 2019 | 49.92102888947 | | 2018 | 49.901798296653 | | 2017 | 49.880995333235 | | 2016 | 49.857736409398 | | 2015 | 49.831230482863 | | 2014 | 49.80059472108 | | 2013 | 49.766097454829 | | 2012 | 49.729969019207 | | 2011 | 49.69550765583 | | 2010 | 49.665266335727 | | 2009 | 49.641223815677 | | 2008 | 49.622446580195 | | 2007 | 49.605324319114 | | 2006 | 49.583619988723 | | 2005 | 49.552637153835 | | 2004 | 49.50956793178 | | 2003 | 49.455306230018 | | 2002 | 49.393412287195 | | 2001 | 49.330001096552 | | 2000 | 49.270093298141 | | 1999 | 49.216083427294 | | 1998 | 49.166967400922 | | 1997 | 49.12040542253 | | 1996 | 49.071833354337 | | 1995 | 49.01756855244 | | 1994 | 48.956068331346 | | 1993 | 48.888407158513 | | 1992 | 48.816864082614 | | 1991 | 48.746129718326 | | 1990 | 48.680232094517 | | 1989 | 48.621084559815 | | 1988 | 48.568601904964 | | 1987 | 48.522290443198 | | 1986 | 48.480373726531 | | 1985 | 48.441650839476 | | 1984 | 48.405812232864 | | 1983 | 48.373639212464 | | 1982 | 48.344598146964 | | 1981 | 48.318380074375 | | 1980 | 48.294491414281 | | 1979 | 48.272566760047 | | 1978 | 48.251797068787 | | 1977 | 48.232314774228 | | 1976 | 48.213729273903 | | 1975 | 48.195544056399 | | 1974 | 48.177879678598 | | 1973 | 48.15987893429 | | 1972 | 48.140222495805 | | 1971 | 48.116812107876 | | 1970 | 48.088374357647 | | 1969 | 48.053589049822 | | 1968 | 48.012564891381 | | 1967 | 47.965932904082 | | 1966 | 47.916063141671 | | 1965 | 47.863571903702 | | 1964 | 47.80911235707 | | 1963 | 47.751747635737 | | 1962 | 47.692014493605 | | 1961 | 47.630506941783 | | 1960 | 47.567153873328 |
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