Year Years 1970 4.36 1971 4.47 1972 4.59 1973 4.71 1974 4.84 1975 4.96 1976 5.09 1977 5.22 1978 5.35 1979 5.48 1980 5.61 1981 5.75 1982 5.89 1983 6.03 1984 6.17 1985 6.32 1986 6.46 1987 6.61 1988 6.76 1989 6.91 1990 7.06 1991 7.21 1992 7.37 1993 7.53 1994 7.68 1995 7.84 1996 8 1997 8.16 1998 8.33 1999 8.49 2000 8.65 2001 8.82 2002 8.98 2003 9.15 2004 9.32 2005 9.48 2006 9.65 2007 9.82 2008 9.99 2009 10.15 2010 10.32 2011 10.49 2012 10.65 2013 10.82 2014 10.99 2015 11.15