| Year | Value | | 2021 | 5.4992454603529 | | 2020 | 5.366080401076 | | 2019 | 5.3514890904206 | | 2018 | 5.8174645065535 | | 2017 | 5.7925181062069 | | 2016 | 6.0603528688248 | | 2015 | 6.1385838577472 | | 2014 | 5.4464226924662 | | 2013 | 5.3922840027643 | | 2012 | 6.0873229970336 | | 2011 | 6.9352681435985 | | 2010 | 8.0511630546497 | | 2009 | 8.3394157466335 | | 2008 | 9.0017842698482 | | 2007 | 8.9474263523423 | | 2006 | 9.8909997598925 | | 2005 | 9.7147361987005 | | 2004 | 13.662593132363 | | 2003 | 16.184584233058 | | 2002 | 33.791875559385 | | 2001 | 32.859134043729 | | 2000 | 46.104550299939 | | 1999 | 41.735106712032 | | 1998 | 38.453450609303 | | 1997 | 37.971299943725 | | 1996 | 38.53117513386 | | 1995 | 38.670741563506 | | 1994 | 40.973802783615 | | 1993 | 47.038763090082 | | 1992 | 48.678761898119 | | 1991 | 46.960100654682 | | 1990 | 43.578387953942 | | 1989 | 42.406003580816 | | 1988 | 43.012123419533 | | 1987 | 42.446014607812 | | 1986 | 29.366899302094 | | 1985 | 33.081422093711 | | 1984 | 30.150081254182 | | 1983 | 30.276994101052 | | 1982 | 26.425106124924 | | 1981 | 24.972766884532 | | 1980 | 32.601521021667 | | 1979 | 36.465545664359 | | 1978 | 39.356159285448 | | 1977 | 35.732916014606 | | 1976 | 35.546651402404 | | 1975 | 30.822391154112 | | 1974 | 39.132495852098 | | 1973 | 42.557918121231 | | 1972 | 43.152924082819 |
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