Year | Value | 2017 | 23.799640177279 | 2016 | 24.352093585977 | 2015 | 22.659509346895 | 2014 | 22.499915923187 | 2013 | 20.370581101524 | 2012 | 21.844685929801 | 2011 | 21.384793370368 | 2010 | 21.667265933799 | 2009 | 22.431079066397 | 2008 | 22.644708184873 | 2007 | 20.895076428172 | 2006 | 20.818328563804 | 2005 | 19.724469990046 | 2004 | 19.592983373077 | 2003 | 19.827064278852 | 2002 | 20.716612714228 | 2001 | 19.362849475307 | 2000 | 19.69909151751 | 1999 | 20.693119987867 | 1998 | 21.174301376924 | 1997 | 20.564868261305 | 1996 | 19.954861343881 | 1995 | 19.221302829282 | 1994 | 19.770179659108 | 1993 | 18.705612977504 | 1992 | 18.481505170025 | 1991 | 19.577350154061 | 1990 | 18.259141126982 |
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