# year value 1960 18.52245 1961 18.81905 1962 18.50208 1963 18.65265 1964 18.41141 1965 18.61765 1966 18.72861 1967 18.63794 1968 18.40753 1969 19.04276 1970 18.80724 1971 18.40161 1972 18.82314 1973 18.82567 1974 18.32224 1975 18.38301 1976 18.38928 1977 18.78381 1978 18.61353 1979 18.62994 1980 18.80302 1981 18.59526 1982 18.76292 1983 19.17493 1984 18.71808 1985 18.70278 1986 18.8342 1987 19.21159 1988 18.90263 1989 18.68931 1990 18.88115 1991 18.91121 1992 18.96535 1993 18.98469 1994 19.07395 1995 19.1259 1996 18.77116 1997 19.17786 1998 19.34813 1999 18.8322 2000 18.82804 2001 19.00172 2002 19.19736 2003 19.20994 2004 19.09384 2005 19.20117 2006 19.13077 2007 18.95991 2008 18.92448 2009 19.20335 2010 19.24901 2011 18.95613 2012 19.17145 2013 19.06132 2014 19.23512 2015 19.47353 2016 19.41251 2017 19.36329 2018 19.16488