Year | Value | 2012 | 20.115487723847 | 2011 | 20.174855935005 | 2010 | 19.199750829309 | 2009 | 20.048161807312 | 2008 | 20.499808225324 | 2007 | 19.067873938373 | 2006 | 18.510715487737 | 2005 | 18.877504716869 | 2004 | 18.672193828289 | 2003 | 18.745264776724 | 2002 | 19.540624658067 | 2001 | 19.597497669027 | 2000 | 19.292368533707 | 1999 | 19.13116645478 | 1998 | 19.18316383043 | 1997 | 18.441033151146 | 1996 | 19.837619254916 | 1995 | 20.485813814377 | 1994 | 20.764142710797 | 1993 | 21.007979267544 | 1992 | 20.499350639875 | 1991 | 20.384410747539 | 1990 | 19.953401074364 | 1989 | 20.151409295196 | 1988 | 20.500889083897 | 1987 | 20.561771184648 | 1986 | 22.516898497291 | 1985 | 21.999629539917 | 1984 | 22.97891585054 | 1983 | 23.067694641944 | 1982 | 22.781200566148 | 1981 | 21.638635271504 | 1980 | 20.991385397543 | 1979 | 21.796714579055 | 1978 | 22.617657715689 | 1977 | 21.269218486245 | 1976 | 19.948266942576 | 1975 | 20.225106236362 | 1974 | 17.864065124693 | 1973 | 18.952488491271 | 1972 | 17.789227166276 |
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