| Year | Value | | 2022 | 30.646599575114 | | 2021 | 30.465176994056 | | 2020 | 30.277091624908 | | 2019 | 30.047189121349 | | 2018 | 29.778858574716 | | 2017 | 29.507318684789 | | 2016 | 29.225307287545 | | 2015 | 28.927601400442 | | 2014 | 28.612392712454 | | 2013 | 28.281138827039 | | 2012 | 27.93540288488 | | 2011 | 27.574897626354 | | 2010 | 27.196914417991 | | 2009 | 27.205427890946 | | 2008 | 27.81590528118 | | 2007 | 28.803751100019 | | 2006 | 30.024103104459 | | 2005 | 30.964264599358 | | 2004 | 31.291332892099 | | 2003 | 31.374718331964 | | 2002 | 31.465205144682 | | 2001 | 31.566893385961 | | 2000 | 31.683648654961 | | 1999 | 31.818013063818 | | 1998 | 31.974407164351 | | 1997 | 32.159057928662 | | 1996 | 32.377605932986 | | 1995 | 32.553570518836 | | 1994 | 32.657628259735 | | 1993 | 32.757533450517 | | 1992 | 32.860779376165 | | 1991 | 32.969757262434 | | 1990 | 33.086265249446 | | 1989 | 33.214795963925 | | 1988 | 33.359822911822 | | 1987 | 33.525539863025 | | 1986 | 33.715803872412 | | 1985 | 33.51532689252 | | 1984 | 32.826861092491 | | 1983 | 32.033530170366 | | 1982 | 31.153241495527 | | 1981 | 30.181152175099 | | 1980 | 29.556884881189 | | 1979 | 29.369410821926 | | 1978 | 29.22652009175 | | 1977 | 29.162876435395 | | 1976 | 29.246129373129 | | 1975 | 29.483027048514 | | 1974 | 29.804555294692 | | 1973 | 30.222445679499 | | 1972 | 30.793031032092 | | 1971 | 31.589519118292 | | 1970 | 32.721648931174 | | 1969 | 34.362179727633 | | 1968 | 36.496521242427 | | 1967 | 37.790746274895 | | 1966 | 37.910459210231 | | 1965 | 38.059345098017 | | 1964 | 38.234778137382 | | 1963 | 38.431652293543 | | 1962 | 38.652041618249 | | 1961 | 38.912216989162 | | 1960 | 39.234482034986 |
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