| Year | Value | | 2015 | 42.663286887285 | | 2014 | 0.59260863925658 | | 2013 | 1.243934704402 | | 2012 | 3.1697662921634 | | 2011 | 3.5279118804698 | | 2010 | 4.2788159596859 | | 2009 | 3.321909723318 | | 2008 | 4.9474677030671 | | 2007 | 5.8355411097118 | | 2006 | 6.8834734368861 | | 2005 | 6.2004789417755 | | 2004 | 6.1622117145146 | | 2003 | 7.0559221735221 | | 2002 | 7.4242182871512 | | 2001 | 9.2460245422805 | | 2000 | 9.2101887672269 | | 1999 | 10.383030760554 | | 1998 | 11.798233322439 | | 1997 | 11.046621575505 | | 1996 | 11.214042333857 | | 1995 | 11.567192556767 | | 1994 | 11.074215330368 | | 1993 | 12.700611982338 | | 1992 | 12.683587500515 | | 1991 | 13.971365664197 | | 1990 | 11.509927787808 | | 1989 | 34.588888088383 | | 1988 | 33.638500994786 | | 1987 | 33.312150993353 | | 1986 | 32.721173745501 | | 1985 | 32.703505009394 | | 1984 | 34.612786641789 | | 1983 | 33.988701390335 | | 1982 | 36.200989847432 | | 1981 | 39.308459157046 | | 1980 | 42.198500439647 | | 1979 | 44.186718313216 | | 1978 | 45.411076319675 | | 1977 | 45.7249500834 | | 1976 | 48.207752545007 | | 1975 | 47.060993295456 | | 1974 | 50.844595565274 | | 1973 | 52.222844396217 | | 1972 | 50.958963761743 | | 1971 | 49.514671462561 | | 1970 | 52.973838243841 | | 1969 | 53.496608626455 | | 1968 | 50.881250193851 | | 1967 | 48.347729198808 | | 1966 | 45.536334186938 | | 1965 | 42.870233150157 | | 1964 | 40.200238945134 | | 1963 | 39.07876185819 | | 1962 | 35.544080981911 | | 1961 | 32.101556184737 | | 1960 | 30.123405013189 |
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