| Year | Value | | 2009 | 7.3584780414868 | | 2008 | 6.1978092880819 | | 2007 | 5.9376876894998 | | 2006 | 6.2176814094673 | | 2005 | 6.6575937153381 | | 2004 | 5.195323351888 | | 2003 | 5.2013253496053 | | 2002 | 4.9943777116483 | | 2001 | 5.7469035584921 | | 2000 | 5.8612820963876 | | 1999 | 8.3278233365821 | | 1998 | 8.6443760539179 | | 1997 | 10.157962098626 | | 1996 | 7.9463151398408 | | 1995 | 6.2207986783915 | | 1994 | 4.3712361162078 | | 1993 | 3.8602459915631 | | 1992 | 6.8811806460261 | | 1991 | 6.1618908119503 | | 1990 | 5.5722365455019 | | 1989 | 5.3697504436685 | | 1988 | 5.4345524231035 | | 1987 | 6.0053384247703 | | 1986 | 6.7842508906153 | | 1985 | 7.3298096379008 | | 1984 | 6.4370965956223 | | 1983 | 6.0339809695025 | | 1982 | 5.9229317268554 | | 1981 | 6.9790355999778 | | 1980 | 5.1702848761984 | | 1979 | 5.9933747422095 | | 1978 | 8.8708585694336 | | 1977 | 8.2163230082934 | | 1976 | 7.5620585041478 | | 1975 | 8.1940784533928 | | 1974 | 5.4177909639063 | | 1973 | 7.5478109107008 | | 1972 | 8.2655132694252 |
|