| Year | Value | | 2018 | 2.14 | | 2017 | 2.21 | | 2016 | 2.34 | | 2015 | 2.44 | | 2014 | 2.54 | | 2013 | 2.59 | | 2012 | 2.66 | | 2011 | 2.7 | | 2010 | 2.73 | | 2009 | 2.76 | | 2008 | 2.81 | | 2007 | 2.86 | | 2006 | 2.89 | | 2005 | 2.93 | | 2004 | 3.01 | | 2003 | 3.05 | | 2002 | 3.13 | | 2001 | 3.27 | | 2000 | 3.58 | | 1999 | 3.7000000477 | | 1998 | 3.7999999523 | | 1997 | 5.2 | | 1996 | 4.4000000954 | | 1995 | 4.9000000954 | | 1994 | 6.5 | | 1993 | 7 | | 1992 | 7.5999999046 | | 1991 | 11.8999996185 | | 1990 | 12.3999996185 | | 1985 | 14.60000038147 | | 1980 | 15.10000038147 | | 1970 | 15.300000190735 | | 1960 | 14.199999809265 |
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