# year    value 
 1960 23.87101 
 1961 24.16954 
 1962 24.02118 
 1963 24.07975 
 1964 23.95709 
 1965 24.08052 
 1966 24.07607 
 1967 24.00255 
 1968 23.85507 
 1969 24.32217 
 1970 24.23963 
 1971 23.80056 
 1972 24.13066 
 1973  24.2665 
 1974 23.56989 
 1975 23.76393 
 1976 23.58675 
 1977 24.05403 
 1978 23.92639 
 1979 24.12302 
 1980  24.2539 
 1981 24.03461 
 1982 24.08578 
 1983 24.41397 
 1984 24.03242 
 1985 24.14679 
 1986 24.22494 
 1987 24.49551 
 1988 24.41666 
 1989   24.033 
 1990 24.35467 
 1991 24.34932 
 1992 24.30161 
 1993 24.31479 
 1994 24.32016 
 1995 24.45523 
 1996 24.33776 
 1997 24.42645 
 1998 24.81779 
 1999 24.29884 
 2000 24.16342 
 2001 24.35056 
 2002 24.65941 
 2003 24.64158 
 2004 24.53717 
 2005 24.83402 
 2006 24.47133 
 2007 24.50292 
 2008 24.37502 
 2009 24.60979 
 2010 24.61429 
 2011 24.29043 
 2012 24.46247 
 2013 24.68098 
 2014  24.7014 
 2015 24.85717 
 2016 24.87516 
 2017 24.79009 
 2018 24.66886