| Year | Value | | 2012 | 8.8345308253276 | | 2011 | 7.7480812707146 | | 2010 | 8.8608193029153 | | 2009 | 8.2618563887212 | | 2008 | 8.2950162829078 | | 2007 | 8.451741147022 | | 2006 | 8.8195707100522 | | 2005 | 9.2064978112902 | | 2003 | 10.816191108162 | | 2002 | 12.152556345367 | | 2001 | 13.922687454794 | | 2000 | 14.482952944491 | | 1999 | 16.545988822095 | | 1998 | 18.464316459151 | | 1997 | 20.056724178681 | | 1996 | 37.323850567115 | | 1995 | 39.619832818046 | | 1994 | 39.962236569619 | | 1993 | 39.55005843397 | | 1992 | 41.294134396355 | | 1991 | 39.469820554649 | | 1990 | 42.217691487883 | | 1989 | 41.08150064683 | | 1988 | 40.011264432554 | | 1987 | 36.297058467498 | | 1986 | 36.432425727222 | | 1985 | 41.055699481865 | | 1984 | 42.986087546658 | | 1983 | 43.533460803059 | | 1982 | 40.073092736409 | | 1981 | 36.131467910282 | | 1980 | 35.454790096878 | | 1979 | 35.641387972994 | | 1978 | 33.161267354931 | | 1977 | 31.981981981982 | | 1976 | 30.601659751037 | | 1975 | 29.187961385576 | | 1974 | 31.28390596745 | | 1973 | 29.330889092576 | | 1972 | 27.277514481306 |
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