| Year | Value | | 2022 | 49.555412015888 | | 2021 | 49.518243603623 | | 2020 | 49.464221408 | | 2019 | 49.38717233228 | | 2018 | 49.284315789819 | | 2017 | 49.162001390565 | | 2016 | 49.001477767177 | | 2015 | 48.815439486365 | | 2014 | 48.708684284571 | | 2013 | 48.680289191317 | | 2012 | 48.650358853842 | | 2011 | 48.617514527922 | | 2010 | 48.580523964018 | | 2009 | 48.54221279713 | | 2008 | 48.505017332819 | | 2007 | 48.470936471505 | | 2006 | 48.441539142273 | | 2005 | 48.419495417208 | | 2004 | 48.403024882358 | | 2003 | 48.388675997149 | | 2002 | 48.378694726228 | | 2001 | 48.364626509208 | | 2000 | 48.340939132677 | | 1999 | 48.317259057987 | | 1998 | 48.295790925281 | | 1997 | 48.273881064698 | | 1996 | 48.254415916778 | | 1995 | 48.236491503189 | | 1994 | 48.222519223721 | | 1993 | 48.210767769406 | | 1992 | 48.200981998155 | | 1991 | 48.182848451459 | | 1990 | 48.148249586452 | | 1989 | 48.110279658208 | | 1988 | 48.069603838877 | | 1987 | 48.023325760239 | | 1986 | 47.972590494098 | | 1985 | 47.915712879053 | | 1984 | 47.853337778115 | | 1983 | 47.78518581369 | | 1982 | 47.704574332722 | | 1981 | 47.609916729522 | | 1980 | 47.505313457496 | | 1979 | 47.405625262338 | | 1978 | 47.323599502763 | | 1977 | 47.252049502903 | | 1976 | 47.180328243728 | | 1975 | 47.108467231912 | | 1974 | 47.038120766113 | | 1973 | 46.969090632145 | | 1972 | 46.900868691055 | | 1971 | 46.808636000777 | | 1970 | 46.712836336764 | | 1969 | 46.640237787097 | | 1968 | 46.568653329958 | | 1967 | 46.496868476144 | | 1966 | 46.425349722837 | | 1965 | 46.354356246465 | | 1964 | 46.286218123098 | | 1963 | 46.220997241767 | | 1962 | 46.160303548197 | | 1961 | 46.104488193653 | | 1960 | 46.043750143893 |
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