| Year | Value | | 2021 | 9.2156825369136 | | 2020 | 9.1771982831524 | | 2019 | 9.1404341381223 | | 2018 | 9.1039105841845 | | 2017 | 9.0187179915636 | | 2016 | 8.9266727197843 | | 2015 | 8.8192777797754 | | 2014 | 8.7438399966087 | | 2013 | 8.6689508278011 | | 2012 | 8.5039745969509 | | 2011 | 8.1271910660721 | | 2010 | 7.9768933157251 | | 2009 | 7.8116699650111 | | 2008 | 7.6196687236228 | | 2007 | 7.5004409243584 | | 2006 | 7.3550094969531 | | 2005 | 7.2747769113492 | | 2004 | 7.0876988561893 | | 2003 | 7.0525048737695 | | 2002 | 6.7803608338521 | | 2001 | 6.6937579553602 | | 2000 | 6.6572249668421 | | 1999 | 6.6525106247391 | | 1998 | 6.6092202419346 | | 1997 | 6.5875972494773 | | 1996 | 6.5280186882211 | | 1995 | 6.3915411590968 | | 1994 | 6.2247900453147 | | 1993 | 6.2049012739258 | | 1992 | 6.0422507583711 | | 1991 | 5.9815584580739 | | 1990 | 5.9212205258381 | | 1989 | 5.8418476943801 | | 1988 | 5.8298563862416 | | 1987 | 5.797817679179 | | 1986 | 5.8059243845716 | | 1985 | 5.7924649592058 | | 1984 | 5.7207364622759 | | 1983 | 5.6939627273386 | | 1982 | 5.6657208802365 | | 1981 | 5.6333278000501 | | 1980 | 5.6363720322453 | | 1979 | 5.6238744767871 | | 1978 | 5.6126931592976 | | 1977 | 5.5410659118317 | | 1976 | 5.52360713264 | | 1975 | 5.4879234448753 | | 1974 | 5.447048214512 | | 1973 | 5.4688576926089 | | 1972 | 5.4591886918657 | | 1971 | 5.4309916582848 | | 1970 | 5.3845197070688 | | 1969 | 5.3717120778121 | | 1968 | 5.3320236140285 | | 1967 | 5.2574052522721 | | 1966 | 5.2313343863938 | | 1965 | 5.2064784734885 | | 1964 | 5.1613733443671 | | 1963 | 5.1221911109889 | | 1962 | 5.0331451826786 | | 1961 | 4.9937604571382 |
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