| Year | Value | | 2021 | 11.006289308176 | | 2020 | 11.006289308176 | | 2019 | 11.006289308176 | | 2018 | 11.006289308176 | | 2017 | 11.006289308176 | | 2016 | 11.006289308176 | | 2015 | 11.006289308176 | | 2014 | 11.006289308176 | | 2013 | 11.006289308176 | | 2012 | 9.4339622641509 | | 2011 | 9.1194968553459 | | 2010 | 9.1194968553459 | | 2009 | 9.1194968553459 | | 2008 | 9.1194968553459 | | 2007 | 9.1194968553459 | | 2006 | 8.8050314465409 | | 2005 | 8.8050314465409 | | 2004 | 8.8050314465409 | | 2003 | 8.8050314465409 | | 2002 | 8.8050314465409 | | 2001 | 8.8050314465409 | | 2000 | 8.8050314465409 | | 1999 | 8.8050314465409 | | 1998 | 9.1194968553459 | | 1997 | 9.1194968553459 | | 1996 | 9.4339622641509 | | 1995 | 9.4339622641509 | | 1994 | 9.4339622641509 | | 1993 | 9.125786163522 | | 1992 | 8.1761006289308 | | 1991 | 7.7044025157233 | | 1990 | 7.6415094339623 | | 1989 | 7.6100628930818 | | 1988 | 7.6100628930818 | | 1987 | 7.5471698113208 | | 1986 | 7.4842767295597 | | 1985 | 7.4842767295597 | | 1984 | 7.2358490566038 | | 1983 | 6.3962264150943 | | 1982 | 6.2578616352201 | | 1981 | 6.1949685534591 | | 1980 | 6.1477987421384 | | 1979 | 6.0534591194969 | | 1978 | 6.0062893081761 | | 1977 | 5.9119496855346 | | 1976 | 5.7861635220126 | | 1975 | 5.7075471698113 | | 1974 | 5.5817610062893 | | 1973 | 5.4716981132075 | | 1972 | 5.377358490566 | | 1971 | 5.3459119496855 | | 1970 | 5.3459119496855 | | 1969 | 5.3459119496855 | | 1968 | 5.3459119496855 | | 1967 | 5.3459119496855 | | 1966 | 5.2830188679245 | | 1965 | 5.2830188679245 | | 1964 | 5.2830188679245 | | 1963 | 5.2830188679245 | | 1962 | 5.2830188679245 | | 1961 | 5.2830188679245 |
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