| Year | Value | | 2014 | 4.41 | | 2013 | 4.51 | | 2012 | 4.64 | | 2011 | 4.7 | | 2010 | 4.76 | | 2009 | 4.83 | | 2008 | 5.01 | | 2007 | 5.06 | | 2006 | 5.09 | | 2005 | 5.12 | | 2004 | 5.14 | | 2003 | 5.31 | | 2002 | 5.53 | | 2001 | 6.15 | | 2000 | 7.04 | | 1999 | 7.4699997902 | | 1998 | 8.3000001907 | | 1997 | 8.6000003815 | | 1996 | 8.4099998474 | | 1995 | 8.6400003433 | | 1994 | 9.6052999496 | | 1993 | 10.7455997467 | | 1992 | 11.9320001602 | | 1991 | 12.0579996109 | | 1990 | 11.9772996902 | | 1989 | 11.97200012207 | | 1988 | 11.947400093079 | | 1987 | 12.064200401306 | | 1986 | 12.030900001526 | | 1985 | 12.030400276184 | | 1984 | 11.977100372314 | | 1983 | 11.936499595642 | | 1982 | 12.103300094604 | | 1981 | 12.118499755859 | | 1980 | 12.011400222778 |
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