| Year | Value | | 2021 | 8.9401243515498 | | 2020 | 7.6784511302434 | | 2019 | 7.4852198389785 | | 2018 | 7.3501426059542 | | 2017 | 7.2061063254265 | | 2016 | 7.0866671132203 | | 2015 | 6.9079676002913 | | 2014 | 6.7640517817465 | | 2013 | 6.6212101396267 | | 2012 | 6.5415901625562 | | 2011 | 6.551375745721 | | 2010 | 6.594329906739 | | 2009 | 6.5751222730693 | | 2008 | 6.8003011899033 | | 2007 | 6.7850986563694 | | 2006 | 6.8610146591638 | | 2005 | 6.8975726956105 | | 2004 | 6.9644270846338 | | 2003 | 7.1255140632809 | | 2002 | 6.9750432223965 | | 2001 | 7.0666179258686 | | 2000 | 7.0875736267272 | | 1999 | 7.1938878809083 | | 1998 | 7.1228479982703 | | 1997 | 7.1550575812136 | | 1996 | 7.0756903354758 | | 1995 | 7.0822481143733 | | 1994 | 7.1174521013635 | | 1993 | 7.1005765491741 | | 1992 | 7.1687434829024 | | 1991 | 7.1281055868049 | | 1990 | 7.1465924686637 | | 1989 | 7.1485410413258 | | 1988 | 7.2644977082526 | | 1987 | 7.3327784713337 | | 1986 | 7.4227519892209 | | 1985 | 7.5388848905162 | | 1984 | 7.5824346626841 | | 1983 | 7.6362842488893 | | 1982 | 7.6361902207349 | | 1981 | 7.7198480656191 | | 1980 | 7.8008891080022 | | 1979 | 7.8225272149237 | | 1978 | 7.8680160982007 | | 1977 | 7.9522409186607 | | 1976 | 7.9759868436109 | | 1975 | 7.9902217168777 | | 1974 | 8.0292452953996 | | 1973 | 8.2212244346397 | | 1972 | 8.3581549785739 | | 1971 | 8.3720094027002 | | 1970 | 8.4340807162639 | | 1969 | 8.5471457029551 | | 1968 | 8.6029993458605 | | 1967 | 8.586697087298 | | 1966 | 8.6964844337769 | | 1965 | 8.8304962664088 | | 1964 | 8.9542266335914 | | 1963 | 9.1070692634078 | | 1962 | 9.3311931046487 | | 1961 | 9.6146247064586 | | 1960 | 9.7174994263675 |
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