| Year | Value | | 2021 | 8.2101893921577 | | 2020 | 7.7232411362463 | | 2019 | 7.3901556895787 | | 2018 | 7.3860402090756 | | 2017 | 7.3479049263648 | | 2016 | 7.3544606318778 | | 2015 | 7.3719728626401 | | 2014 | 7.3796732041165 | | 2013 | 7.3860444075196 | | 2012 | 7.4178208186741 | | 2011 | 7.4268254070921 | | 2010 | 7.4583922174339 | | 2009 | 7.4473289871415 | | 2008 | 7.4578418418341 | | 2007 | 7.3898151920627 | | 2006 | 7.3474573555293 | | 2005 | 7.2416925505834 | | 2004 | 7.2055004958718 | | 2003 | 7.2245278382252 | | 2002 | 7.2567291624814 | | 2001 | 7.2270526316843 | | 2000 | 7.2394799218639 | | 1999 | 7.198303905024 | | 1998 | 7.1528739605281 | | 1997 | 7.2037097114635 | | 1996 | 7.303043480008 | | 1995 | 7.3929709742097 | | 1994 | 7.3592901510926 | | 1993 | 7.3721402884319 | | 1992 | 7.2143367887479 | | 1991 | 7.2048915587306 | | 1990 | 7.1852687601607 | | 1989 | 7.0807356743647 | | 1988 | 7.18986133577 | | 1987 | 7.2389431586548 | | 1986 | 7.3494401425678 | | 1985 | 7.4508145783688 | | 1984 | 7.5176467437054 | | 1983 | 7.5555836528909 | | 1982 | 7.3728349986466 | | 1981 | 7.271690742653 | | 1980 | 7.347723670878 | | 1979 | 7.2680378903292 | | 1978 | 7.3261528230877 | | 1977 | 7.7253799182674 | | 1976 | 8.0321722862907 | | 1975 | 8.0661567365594 | | 1974 | 8.1785299193271 | | 1973 | 8.0319936491732 | | 1972 | 8.4730497561125 | | 1971 | 8.277636379195 | | 1970 | 8.4976738597279 | | 1969 | 8.8253246234014 | | 1968 | 8.9619789458079 | | 1967 | 9.0646673071671 | | 1966 | 9.3178698790197 | | 1965 | 9.7801191119252 | | 1964 | 11.007067252808 | | 1963 | 10.246011221439 | | 1962 | 10.320019352569 | | 1961 | 12.870600218074 | | 1960 | 19.813337972957 |
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