| Year | Value | | 2021 | 32.298502591326 | | 2020 | 32.11087456417 | | 2019 | 32.104262189077 | | 2018 | 32.098942165634 | | 2017 | 32.09431972254 | | 2016 | 32.09868457419 | | 2015 | 32.070233090797 | | 2014 | 32.029948415128 | | 2013 | 31.990679853631 | | 2012 | 31.948692681441 | | 2011 | 31.909073612312 | | 2010 | 31.869510058024 | | 2009 | 31.847012290014 | | 2008 | 31.82392255622 | | 2007 | 31.784754845598 | | 2006 | 31.76085493096 | | 2005 | 31.736315126959 | | 2004 | 31.712203095703 | | 2003 | 31.687871959636 | | 2002 | 31.664389808941 | | 2001 | 31.638080153091 | | 2000 | 31.613867591262 | | 1999 | 31.606973914071 | | 1998 | 31.586984551836 | | 1997 | 31.566902491107 | | 1996 | 31.546847800063 | | 1995 | 31.526838535019 | | 1994 | 31.506864851689 | | 1993 | 31.486887178323 | | 1992 | 31.460489526361 | | 1991 | 31.646718209758 | | 1990 | 31.628694607231 |
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