| Year | Value | | 2021 | 13.973316491671 | | 2020 | 13.973316491671 | | 2019 | 13.973316491671 | | 2018 | 13.973316491671 | | 2017 | 13.973316491671 | | 2016 | 13.815425909844 | | 2015 | 13.262808873451 | | 2014 | 13.262808873451 | | 2013 | 13.262808873451 | | 2012 | 12.55230125523 | | 2011 | 12.31546538249 | | 2010 | 11.920738927923 | | 2009 | 11.762848346096 | | 2008 | 11.778637404279 | | 2007 | 11.782584668824 | | 2006 | 11.160495776427 | | 2005 | 11.149443435699 | | 2004 | 11.135233283335 | | 2003 | 11.091813373332 | | 2002 | 11.02076261151 | | 2001 | 11.028657140602 | | 2000 | 11.032604405147 | | 1999 | 11.039709481329 | | 1998 | 11.039709481329 | | 1997 | 11.040498934239 | | 1996 | 11.040498934239 | | 1995 | 11.040498934239 | | 1994 | 10.251046025105 | | 1993 | 10.251835478014 | | 1992 | 10.252624930923 | | 1991 | 9.546064577248 | | 1990 | 8.7124023052025 | | 1989 | 7.5740112102313 | | 1988 | 7.4911186547722 | | 1987 | 7.501381542591 | | 1986 | 7.7129549222389 | | 1985 | 7.7129549222389 | | 1984 | 7.7145338280572 | | 1983 | 7.896107997158 | | 1982 | 7.7776900607879 | | 1981 | 7.7145338280572 | | 1980 | 8.0618931080761 | | 1979 | 7.8566353517013 | | 1978 | 7.7161127338754 | | 1977 | 7.7161127338754 | | 1976 | 7.7161127338754 | | 1975 | 7.7176916396937 | | 1974 | 7.7176916396937 | | 1973 | 8.5339859477382 | | 1972 | 8.8205573537539 | | 1971 | 8.862398357938 | | 1970 | 8.8395042235731 | | 1969 | 9.6100102628878 | | 1968 | 9.6115891687061 | | 1967 | 9.0771295492224 | | 1966 | 9.0771295492224 | | 1965 | 9.0771295492224 | | 1964 | 9.0779190021315 | | 1963 | 9.0779190021315 | | 1962 | 9.0779190021315 | | 1961 | 9.0779190021315 |
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