# year    value 
 1979 27.42021 
 1980 27.41805 
 1981 27.36419 
 1982  27.3083 
 1983 27.66303 
 1984 27.02922 
 1985 27.02069 
 1986 27.27598 
 1987 27.87882 
 1988 27.49253 
 1989 27.38972 
 1990 27.56394 
 1991 27.47408 
 1992 27.34182 
 1993 27.58007 
 1994  27.3656 
 1995 27.79684 
 1996  27.4252 
 1997 27.74404 
 1998  28.0964 
 1999 27.48845 
 2000 27.47394 
 2001 27.60805 
 2002 27.90983 
 2003 27.74372 
 2004 27.65653 
 2005 27.93892 
 2006 27.95401 
 2007 27.79832 
 2008 27.73396 
 2009 28.23174 
 2010 28.18444 
 2011 28.07289 
 2012 27.97204 
 2013 27.80593 
 2014 28.19574 
 2015 28.32975 
 2016 28.23078 
 2017 28.27223 
 2018 28.13897