| Year | Value | | 2012 | 0.3 | | 2006 | 5 | | 2002 | 0.15 | | 2001 | 0.2 | | 1999 | 0.200000003 | | 1997 | 0.1700000018 | | 1996 | 0.1599999964 | | 1995 | 0.1700000018 | | 1994 | 0.2322999984 | | 1993 | 0.2387000024 | | 1992 | 0.2425999939 | | 1991 | 0.2372000068 | | 1990 | 0.243599996 | | 1989 | 0.24429999291897 | | 1988 | 0.2398000061512 | | 1987 | 0.22699999809265 | | 1985 | 0.18019999563694 | | 1980 | 0.1756000071764 | | 1975 | 0.13770000636578 | | 1970 | 0.13529999554157 | | 1960 | 0.11576379835606 |
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