# year    value 
 1960  19.9144 
 1961 19.70278 
 1962 19.69162 
 1963 19.83334 
 1964 19.60584 
 1965 19.67045 
 1966  19.8771 
 1967  19.4751 
 1968 19.48339 
 1969 19.98691 
 1970 19.74584 
 1971 19.45872 
 1972  19.8005 
 1973 19.96557 
 1974 19.59013 
 1975 19.65199 
 1976 19.56989 
 1977 19.85546 
 1978 19.87376 
 1979 20.02618 
 1980  20.1357 
 1981  19.8903 
 1982 19.85074 
 1983 19.93025 
 1984 19.78225 
 1985 19.82801 
 1986 19.84779 
 1987 20.19812 
 1988 20.09091 
 1989 19.79413 
 1990 20.13201 
 1991 20.07195 
 1992 19.75656 
 1993 19.99561 
 1994 20.16754 
 1995 20.18402 
 1996 20.07678 
 1997 20.05903 
 1998 20.68103 
 1999 20.27026 
 2000 20.13733 
 2001 20.36044 
 2002 20.49706 
 2003 20.50195 
 2004 20.35605 
 2005 20.48141 
 2006 20.52677 
 2007 20.37329 
 2008 20.18721 
 2009 20.72583 
 2010 20.68428 
 2011 20.37556 
 2012 20.44434 
 2013 20.46788 
 2014  20.4854 
 2015 20.69613 
 2016 20.85143 
 2017 20.67824 
 2018 20.55476