| Year | Value | | 2020 | 5.533122811554 | | 2019 | 5.6453675017195 | | 2018 | 5.6746968827193 | | 2017 | 5.1035181850095 | | 2016 | 4.0829412464118 | | 2015 | 5.7298792926408 | | 2014 | 6.1414210947228 | | 2013 | 5.8029720429004 | | 2012 | 5.9546462575182 | | 2011 | 5.9583431065294 | | 2010 | 5.5547766489411 | | 2009 | 5.2643961059476 | | 2008 | 6.1210193354514 | | 2007 | 6.9963220681665 | | 2006 | 6.6980907607014 | | 2005 | 6.8299138555045 | | 2004 | 7.0687104037517 | | 2003 | 6.4208891244683 | | 2002 | 5.8446236150074 | | 2001 | 5.5979078536161 | | 2000 | 4.5279858100118 | | 1999 | 5.0034896873722 | | 1998 | 5.3026779187986 | | 1997 | 5.2056453736374 | | 1996 | 5.130915882581 | | 1995 | 5.2611218568665 | | 1994 | 6.0022650056625 | | 1993 | 5.8705512909979 | | 1992 | 8.7254318046456 | | 1991 | 10.350276534106 | | 1990 | 11.350737797957 | | 1989 | 11.026660890797 | | 1988 | 12.401958592318 | | 1987 | 13.462836118535 | | 1986 | 14.841091191682 | | 1985 | 12.692050768203 | | 1984 | 14.042926673582 | | 1983 | 10.50825729583 | | 1982 | 15.711157280027 | | 1981 | 17.038224537849 | | 1980 | 20.004111842105 | | 1979 | 20.314805705853 | | 1978 | 12.091158472252 | | 1977 | 14.851485148515 | | 1976 | 12.127659574468 | | 1975 | 9.9173553719008 | | 1974 | 5.0788091068301 | | 1973 | 4.2253521126761 | | 1972 | 5.6603773584906 |
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