| Year | Value | | 2021 | 7.9789565979833 | | 2020 | 7.8912757562473 | | 2019 | 7.628233231039 | | 2018 | 7.5405523893029 | | 2017 | 7.5405523893029 | | 2016 | 7.4528715475669 | | 2015 | 7.4528715475669 | | 2014 | 7.0144673388865 | | 2013 | 7.0144673388865 | | 2012 | 7.0144673388865 | | 2011 | 6.8829460762823 | | 2010 | 6.8829460762823 | | 2009 | 6.7952652345462 | | 2008 | 6.6637439719421 | | 2007 | 6.6637439719421 | | 2006 | 6.6637439719421 | | 2005 | 6.6637439719421 | | 2004 | 6.6637439719421 | | 2003 | 6.6637439719421 | | 2002 | 6.6637439719421 | | 2001 | 6.532222709338 | | 2000 | 6.532222709338 | | 1999 | 6.576063130206 | | 1998 | 6.48838228847 | | 1997 | 6.48838228847 | | 1996 | 6.4445418676019 | | 1995 | 6.4007014467339 | | 1994 | 6.2691801841298 | | 1993 | 6.2691801841298 | | 1992 | 5.9622972380535 | | 1991 | 5.6992547128452 | | 1990 | 5.5238930293731 | | 1989 | 5.2608505041648 | | 1988 | 5.0416483998246 | | 1987 | 4.9101271372205 | | 1986 | 4.5155633494081 | | 1985 | 4.4278825076721 | | 1984 | 4.2525208241999 | | 1983 | 4.2525208241999 | | 1982 | 4.2525208241999 | | 1981 | 4.2525208241999 | | 1980 | 4.2086804033319 | | 1979 | 4.1209995615958 | | 1978 | 4.0771591407278 | | 1977 | 3.7702761946515 | | 1976 | 3.7264357737834 | | 1975 | 3.7264357737834 | | 1974 | 3.6825953529154 | | 1973 | 3.6825953529154 | | 1972 | 3.6387549320474 | | 1971 | 3.6387549320474 | | 1970 | 3.5949145111793 | | 1969 | 3.5949145111793 | | 1968 | 3.5510740903113 | | 1967 | 3.5510740903113 | | 1966 | 3.5510740903113 | | 1965 | 3.5072336694432 | | 1964 | 3.5072336694432 | | 1963 | 3.5072336694432 | | 1962 | 3.4633932485752 | | 1961 | 3.4633932485752 |
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