| Year | Value | | 2015 | 38.377996732738 | | 2014 | 42.386617370181 | | 2013 | 40.305912345913 | | 2012 | 43.890296980075 | | 2011 | 43.761179751215 | | 2010 | 41.158572270398 | | 2009 | 34.477580645906 | | 2008 | 44.391399986064 | | 2007 | 40.061535309824 | | 2005 | 44.628054821847 | | 2004 | 32.121605685704 | | 2003 | 32.728431991396 | | 2002 | 34.747382453995 | | 2001 | 33.840138640312 | | 2000 | 41.657100929298 | | 1999 | 32.311477101111 | | 1998 | 25.245624309145 | | 1997 | 41.447285398095 | | 1996 | 35.55973133789 | | 1995 | 32.682488388648 | | 1994 | 29.140432314449 | | 1993 | 17.46450265368 | | 1992 | 19.325823542119 | | 1991 | 28.780449538945 | | 1990 | 24.252128840792 | | 1989 | 22.292774246458 | | 1988 | 21.410559582839 | | 1987 | 12.675656543258 | | 1986 | 15.292271140628 | | 1985 | 37.126957678933 | | 1984 | 43.681403417669 | | 1983 | 41.631200185886 | | 1982 | 46.651526619747 | | 1981 | 48.842305857142 | | 1980 | 47.831232034002 | | 1979 | 38.180266836945 | | 1978 | 38.584940654655 |
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