Year Years 1970 5.64 1971 5.78 1972 5.92 1973 6.07 1974 6.22 1975 6.37 1976 6.52 1977 6.68 1978 6.84 1979 7 1980 7.16 1981 7.32 1982 7.49 1983 7.66 1984 7.83 1985 8 1986 8.18 1987 8.35 1988 8.53 1989 8.71 1990 8.89 1991 9.07 1992 9.26 1993 9.44 1994 9.63 1995 9.81 1996 10 1997 10.19 1998 10.38 1999 10.56 2000 10.74 2001 10.92 2002 11.1 2003 11.28 2004 11.46 2005 11.63 2006 11.8 2007 11.97 2008 12.14 2009 12.31 2010 12.47 2011 12.64 2012 12.8 2013 12.96 2014 13.11 2015 13.27