| Year | Value | | 2021 | 61.25304716704 | | 2020 | 61.76175891348 | | 2019 | 62.26889952651 | | 2018 | 61.971353180363 | | 2017 | 61.656536220661 | | 2016 | 61.291998998699 | | 2015 | 61.014309073599 | | 2014 | 60.709987355387 | | 2013 | 60.466013886947 | | 2012 | 60.166584242796 | | 2011 | 59.966142083292 | | 2010 | 59.307740958904 | | 2009 | 59.061372229522 | | 2008 | 58.01722426374 | | 2007 | 58.007442460904 | | 2006 | 57.540198545243 | | 2005 | 57.054899610091 | | 2004 | 56.661969620333 | | 2003 | 56.415203936299 | | 2002 | 56.145309651683 | | 2001 | 55.725110511227 | | 2000 | 55.505903805196 | | 1999 | 54.876159496814 | | 1998 | 54.160694272297 | | 1997 | 54.374324880243 | | 1996 | 54.077267474557 | | 1995 | 53.999518003987 | | 1994 | 54.284684925033 | | 1993 | 53.669021426245 | | 1992 | 53.277820942241 | | 1991 | 53.304959840525 | | 1990 | 53.298778900632 | | 1989 | 53.279336638992 | | 1988 | 52.246558124171 | | 1987 | 52.675633272713 | | 1986 | 52.227526767793 | | 1985 | 51.603952195665 | | 1984 | 51.430585116685 | | 1983 | 51.578687821814 | | 1982 | 52.184855425367 | | 1981 | 52.206431230972 | | 1980 | 52.073318538897 | | 1979 | 51.850547815483 | | 1978 | 51.606901670607 | | 1977 | 51.47583423535 | | 1976 | 50.945805687981 | | 1975 | 50.4822685841 | | 1974 | 50.235853361657 | | 1973 | 50.102001562848 | | 1972 | 49.505882989449 | | 1971 | 49.49907994905 | | 1970 | 48.924549034427 | | 1969 | 48.541163385146 | | 1968 | 48.439151608903 | | 1967 | 48.0138422853 | | 1966 | 47.775660018203 | | 1965 | 47.805208793853 | | 1964 | 47.697373900761 | | 1963 | 47.18477637525 | | 1962 | 46.882616634092 | | 1961 | 46.539384904217 | | 1960 | 46.225204326495 |
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