# year value 1960 14.82234 1961 15.13301 1962 15.07743 1963 14.89235 1964 14.4498 1965 15.04976 1966 14.68625 1967 15.04714 1968 14.95292 1969 15.0283 1970 14.94608 1971 14.76291 1972 15.05462 1973 14.99284 1974 14.83233 1975 15.0304 1976 14.58845 1977 15.27052 1978 15.0644 1979 15.2494 1980 15.44176 1981 15.29279 1982 15.32014 1983 15.20006 1984 14.62698 1985 15.29866 1986 15.0344 1987 15.19167 1988 15.22297 1989 15.32587 1990 15.29556 1991 15.29695 1992 14.78121 1993 15.17894 1994 15.40525 1995 14.98544 1996 15.11105 1997 15.36293 1998 15.36285 1999 15.36429 2000 15.085 2001 15.28293 2002 15.23877 2003 15.38747 2004 15.4213 2005 15.384 2006 15.46645 2007 15.26757 2008 15.48061 2009 15.65001 2010 15.31576 2011 15.49416 2012 15.53919 2013 15.78732 2014 15.80162 2015 15.68403 2016 15.55662 2017 15.8642 2018 15.7798