| Year | Value | | 2022 | 51.67322766005 | | 2021 | 51.710971486293 | | 2020 | 51.781414463363 | | 2019 | 51.85153215837 | | 2018 | 51.881908079575 | | 2017 | 51.895513524319 | | 2016 | 51.888981251595 | | 2015 | 51.845018247624 | | 2014 | 51.804981126695 | | 2013 | 51.794745211225 | | 2012 | 51.786067852249 | | 2011 | 51.771344278455 | | 2010 | 51.756764531652 | | 2009 | 51.718920651393 | | 2008 | 51.632781093749 | | 2007 | 51.525466994012 | | 2006 | 51.421006076227 | | 2005 | 51.35124690586 | | 2004 | 51.336905215889 | | 2003 | 51.344683903862 | | 2002 | 51.344158585387 | | 2001 | 51.341068612518 | | 2000 | 51.337529563491 | | 1999 | 51.329946347321 | | 1998 | 51.321378919031 | | 1997 | 51.304492804281 | | 1996 | 51.277075084414 | | 1995 | 51.260524845806 | | 1994 | 51.247392803588 | | 1993 | 51.229692168925 | | 1992 | 51.205929745748 | | 1991 | 51.166172690048 | | 1990 | 51.134598562661 | | 1989 | 51.117823545761 | | 1988 | 51.093753527528 | | 1987 | 51.073384579065 | | 1986 | 51.044963795607 | | 1985 | 51.012784100905 | | 1984 | 50.983150185724 | | 1983 | 50.960229918462 | | 1982 | 50.935988248164 | | 1981 | 50.902363535393 | | 1980 | 50.853126122045 | | 1979 | 50.793143882096 | | 1978 | 50.72917171744 | | 1977 | 50.661545407471 | | 1976 | 50.602449496781 | | 1975 | 50.555792511484 | | 1974 | 50.521409853377 | | 1973 | 50.506148064751 | | 1972 | 50.499100501553 | | 1971 | 50.489063858787 | | 1970 | 50.478904455548 | | 1969 | 50.471331425776 | | 1968 | 50.465031862161 | | 1967 | 50.463532527944 | | 1966 | 50.467600136811 | | 1965 | 50.455350260221 | | 1964 | 50.425421822081 | | 1963 | 50.399045986636 | | 1962 | 50.376844879462 | | 1961 | 50.354839980323 | | 1960 | 50.343789137392 |
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