| Year | Value | | 2021 | 24.657472711329 | | 2020 | 24.492153666554 | | 2019 | 24.326861520419 | | 2018 | 24.161529026323 | | 2017 | 23.996196532227 | | 2016 | 23.830864038132 | | 2015 | 23.665531544036 | | 2014 | 23.431292802462 | | 2013 | 23.197054060888 | | 2012 | 22.962815319314 | | 2011 | 22.72857657774 | | 2010 | 22.494337836166 | | 2009 | 22.372191109461 | | 2008 | 22.250044382757 | | 2007 | 22.127897656053 | | 2006 | 22.005750929348 | | 2005 | 21.883604202644 | | 2004 | 21.761457475939 | | 2003 | 21.639310749235 | | 2002 | 21.51716402253 | | 2001 | 21.395017295826 | | 2000 | 21.272870569121 | | 1999 | 21.19607360544 | | 1998 | 21.119276641758 | | 1997 | 21.042479678077 | | 1996 | 20.965682714396 | | 1995 | 20.888885750714 | | 1994 | 20.812088787033 | | 1993 | 20.735291823351 | | 1992 | 20.65849485967 | | 1991 | 20.581697895988 | | 1990 | 20.504900932307 |
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