| Year | Value | | 2021 | 7.6166082502022 | | 2020 | 7.6166082502022 | | 2019 | 7.6166082502022 | | 2018 | 7.6166082502022 | | 2017 | 7.6227738801943 | | 2016 | 7.6227738801943 | | 2015 | 7.6227738801943 | | 2014 | 7.6227738801943 | | 2013 | 7.6002152273339 | | 2012 | 7.6002152273339 | | 2011 | 7.6002152273339 | | 2010 | 7.6525423728814 | | 2009 | 7.2639225181598 | | 2008 | 7.2639225181598 | | 2007 | 7.2639225181598 | | 2006 | 7.2639225181598 | | 2005 | 7.3715361850955 | | 2004 | 7.3715361850955 | | 2003 | 7.3715361850955 | | 2002 | 7.3715361850955 | | 2001 | 7.3715361850955 | | 2000 | 7.3715361850955 | | 1999 | 7.1294054344902 | | 1998 | 6.9948883508205 | | 1997 | 6.8603712671509 | | 1996 | 6.7258541834813 | | 1995 | 6.7258541834813 | | 1994 | 6.7258541834813 | | 1993 | 6.7258541834813 | | 1992 | 6.7258541834813 | | 1991 | 6.7124024751143 | | 1990 | 6.7124024751143 | | 1989 | 6.5913370998117 | | 1988 | 6.456820016142 | | 1987 | 6.3895614743072 | | 1986 | 6.3223029324724 | | 1985 | 6.2550443906376 | | 1984 | 6.1877858488028 | | 1983 | 6.120527306968 | | 1982 | 6.0532687651332 | | 1981 | 5.9187516814635 | | 1980 | 5.8514931396287 | | 1979 | 5.8380414312618 | | 1978 | 5.8380414312618 | | 1977 | 5.8380414312618 | | 1976 | 5.8245897228948 | | 1975 | 5.8245897228948 | | 1974 | 5.8245897228948 | | 1973 | 5.8111380145278 | | 1972 | 5.8111380145278 | | 1971 | 5.8111380145278 | | 1970 | 5.8111380145278 | | 1969 | 5.8111380145278 | | 1968 | 5.8380414312618 | | 1967 | 5.8380414312618 | | 1966 | 5.8918482647296 | | 1965 | 5.8918482647296 | | 1964 | 5.8918482647296 | | 1963 | 5.8918482647296 | | 1962 | 5.8918482647296 | | 1961 | 5.8918482647296 |
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