| Year | Value | | 2021 | 9.9283797340967 | | 2020 | 9.7719109912899 | | 2019 | 8.6249700659685 | | 2018 | 8.6376362968736 | | 2017 | 8.5718596360072 | | 2016 | 8.3945196873394 | | 2015 | 8.4075490052542 | | 2014 | 8.1187011069856 | | 2013 | 8.1732121023093 | | 2012 | 8.1542081043449 | | 2011 | 8.0488793767431 | | 2010 | 7.9998480820406 | | 2009 | 7.9469365847615 | | 2008 | 8.0124232530905 | | 2007 | 7.9362795691658 | | 2006 | 7.8926345471267 | | 2005 | 8.0131212597466 | | 2004 | 7.8971713386165 | | 2003 | 8.1129194022089 | | 2002 | 8.0647947128783 | | 2001 | 8.0486548185355 | | 2000 | 8.1108779026587 | | 1999 | 8.2747433078747 | | 1998 | 8.2400196264518 | | 1997 | 8.2584450934268 | | 1996 | 8.3375089736962 | | 1995 | 8.4141450983956 | | 1994 | 8.3813994006148 | | 1993 | 8.4795481281291 | | 1992 | 8.3611402287538 | | 1991 | 8.4625074834586 | | 1990 | 8.5001841797353 | | 1989 | 8.5179985075339 | | 1988 | 8.5536114142712 | | 1987 | 8.4958405829694 | | 1986 | 8.6175905163451 | | 1985 | 8.7282616951466 | | 1984 | 8.6536526259139 | | 1983 | 8.7660759297597 | | 1982 | 8.6857489848164 | | 1981 | 8.8023852264074 | | 1980 | 8.9953948296234 | | 1979 | 8.8628680897106 | | 1978 | 9.0101769780118 | | 1977 | 8.9800083519093 | | 1976 | 9.2253691210379 | | 1975 | 9.2664633574211 | | 1974 | 9.3177389016832 | | 1973 | 9.4814864507975 | | 1972 | 9.4434459470415 | | 1971 | 9.5528614987372 | | 1970 | 9.7062664929937 | | 1969 | 9.8298328001102 | | 1968 | 9.8616835991428 | | 1967 | 9.6536469829221 | | 1966 | 9.7553985290729 | | 1965 | 9.8659866941051 | | 1964 | 9.8043268773759 | | 1963 | 10.104347239453 | | 1962 | 10.190298345374 | | 1961 | 10.045318965959 | | 1960 | 10.239991399976 |
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