Year | Value | 2021 | 18.867988645988 | 2020 | 18.821054816231 | 2019 | 18.774103393574 | 2018 | 18.727171452985 | 2017 | 18.68010843249 | 2016 | 18.628349129809 | 2015 | 18.586346703339 | 2014 | 18.532816734288 | 2013 | 18.482469580987 | 2012 | 18.432122427686 | 2011 | 18.381775274385 | 2010 | 18.331428121084 | 2009 | 18.296356593356 | 2008 | 18.261285086588 | 2007 | 18.226213558859 | 2006 | 18.191142031131 | 2005 | 18.156070503403 | 2004 | 18.120998975675 | 2003 | 18.079549703204 | 2002 | 18.04449054295 | 2001 | 18.009431382697 | 2000 | 17.974372222443 | 1999 | 17.908603078612 | 1998 | 17.841515690613 | 1997 | 17.774428302614 | 1996 | 17.707340914615 | 1995 | 17.640253505662 | 1994 | 17.573166117663 | 1993 | 17.480719986157 | 1992 | 17.413729778794 | 1991 | 17.346739571431 | 1990 | 17.279749364068 |
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