# year value 1979 25.55152 1980 25.59856 1981 25.39654 1982 25.21069 1983 25.43602 1984 25.24197 1985 25.29956 1986 25.33096 1987 25.87991 1988 25.67044 1989 25.31871 1990 25.76807 1991 25.57821 1992 25.24251 1993 25.55816 1994 25.53467 1995 25.68718 1996 25.64035 1997 25.64496 1998 26.16019 1999 25.70552 2000 25.60048 2001 25.85249 2002 25.9765 2003 26.00502 2004 25.95023 2005 26.10741 2006 26.03289 2007 25.991 2008 25.81759 2009 26.24624 2010 26.57264 2011 25.93707 2012 26.00713 2013 26.10028 2014 26.16998 2015 26.30743 2016 26.56242 2017 26.34488 2018 26.3235