# year    value 
 1960 14.61635 
 1961 14.74117 
 1962 14.79438 
 1963 14.76936 
 1964 14.41428 
 1965 14.50454 
 1966 14.55418 
 1967 14.64264 
 1968 14.48332 
 1969 14.57753 
 1970  14.6587 
 1971 14.53204 
 1972 14.39683 
 1973 14.91838 
 1974 14.37778 
 1975 14.71627 
 1976 14.32294 
 1977 14.81113 
 1978 14.59551 
 1979 14.71885 
 1980 14.85287 
 1981 15.13155 
 1982  14.6788 
 1983 15.07457 
 1984 14.67619 
 1985 14.67651 
 1986 14.82904 
 1987 15.01067 
 1988  15.1356 
 1989 14.92151 
 1990 15.22675 
 1991 15.11381 
 1992 14.78409 
 1993 14.91249 
 1994 15.03061 
 1995 15.33737 
 1996 14.92679 
 1997 15.17767 
 1998 15.54389 
 1999 15.33122 
 2000 15.18918 
 2001 15.39041 
 2002  15.4751 
 2003 15.47537 
 2004 15.35802 
 2005 15.67858 
 2006 15.54809 
 2007  15.6189 
 2008 15.41044 
 2009 15.43016 
 2010 15.69115 
 2011 15.45784 
 2012 15.52596 
 2013  15.5209 
 2014 15.52765 
 2015 15.80197 
 2016 15.97373 
 2017 15.80413 
 2018 15.56401