| Year | Value | | 2021 | 8.350502372236 | | 2020 | 7.4268731618302 | | 2019 | 7.2165563310364 | | 2018 | 7.2694348104455 | | 2017 | 7.2645647865202 | | 2016 | 7.3970802136073 | | 2015 | 7.5657178398491 | | 2014 | 7.726252053468 | | 2013 | 7.8681807392875 | | 2012 | 8.0527084580972 | | 2011 | 8.2377259729896 | | 2010 | 8.4336827643245 | | 2009 | 8.722392984045 | | 2008 | 8.9710428847954 | | 2007 | 9.2005110655558 | | 2006 | 9.4827544396312 | | 2005 | 9.6849942536114 | | 2004 | 9.7837830520037 | | 2003 | 9.880902834175 | | 2002 | 9.8419455802881 | | 2001 | 9.7920162155388 | | 2000 | 9.7339242889732 | | 1999 | 9.9253506219222 | | 1998 | 9.7499921795287 | | 1997 | 9.6020582334516 | | 1996 | 9.4552001376602 | | 1995 | 9.4525589914757 | | 1994 | 9.4767154175821 | | 1993 | 9.4060070480035 | | 1992 | 9.4147693326216 | | 1991 | 9.4172935129571 | | 1990 | 9.4348477676314 | | 1989 | 9.4731476382551 | | 1988 | 9.8398487127757 | | 1987 | 9.9515500435702 | | 1986 | 10.132697417196 | | 1985 | 10.364820511127 | | 1984 | 10.522199589504 | | 1983 | 10.751353685191 | | 1982 | 10.869951142872 | | 1981 | 11.12724711928 | | 1980 | 11.308434835287 | | 1979 | 11.740519469093 | | 1978 | 11.960194040358 | | 1977 | 11.958920437298 | | 1976 | 12.192908954535 | | 1975 | 12.266965646138 | | 1974 | 12.008837230431 | | 1973 | 11.954203206881 | | 1972 | 12.161530948703 | | 1971 | 12.442635614703 | | 1970 | 12.797540664774 | | 1969 | 13.026769244755 | | 1968 | 12.926928528222 | | 1967 | 13.045035614911 | | 1966 | 13.280638462932 | | 1965 | 13.509417820062 | | 1964 | 13.816776746416 | | 1963 | 14.159102332425 | | 1962 | 14.26950808581 | | 1961 | 14.56340082533 | | 1960 | 14.888118207758 |
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