| Year | Value | | 2022 | 3.1721370549075 | | 2021 | 3.553882996921 | | 2020 | 4.0456495098039 | | 2019 | 4.0298610155498 | | 2018 | 4.04108625299 | | 2017 | 4.3207377417904 | | 2016 | 4.6706353135314 | | 2015 | 4.6441113490364 | | 2014 | 4.4185120280389 | | 2013 | 4.1413976297507 | | 2012 | 3.8444905725653 | | 2011 | 3.5914972273567 | | 2010 | 3.2777559421609 | | 2009 | 3.6105352496765 | | 2008 | 2.8001613687379 | | 2007 | 2.9569902343439 | | 2006 | 3.1029477284813 | | 2005 | 3.2143810953301 | | 2004 | 4.0449288479601 | | 2003 | 4.6588328030263 | | 2002 | 4.6111066063505 | | 2001 | 4.14808163199 | | 2000 | 3.9616709688485 | | 1999 | 5.5036215436654 | | 1998 | 5.3331039525182 | | 1997 | 5.0684874125581 | | 1996 | 5.2739397637624 | | 1995 | 5.1832317904883 | | 1994 | 5.1113021878284 | | 1993 | 5.3698551673349 | | 1992 | 5.8780719923865 | | 1991 | 5.7037506481535 | | 1990 | 5.2847915443335 | | 1989 | 5.6446616644868 | | 1988 | 5.603045758207 | | 1987 | 5.2532538811353 | | 1986 | 5.8377624814847 | | 1985 | 4.5881581822154 | | 1984 | 4.1793056959892 | | 1983 | 4.7903360177728 | | 1982 | 8.5351619492267 | | 1981 | 6.9025815655055 | | 1980 | 5.6988913066004 | | 1979 | 5.8927519151444 | | 1978 | 4.95813133539 | | 1977 | 2.0779220779221 | | 1976 | 1.7522872403161 | | 1975 | 1.5261798544259 |
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