Year Years 1970 4.47 1971 4.59 1972 4.71 1973 4.84 1974 4.97 1975 5.1 1976 5.23 1977 5.37 1978 5.51 1979 5.65 1980 5.79 1981 5.94 1982 6.09 1983 6.24 1984 6.39 1985 6.54 1986 6.7 1987 6.86 1988 7.02 1989 7.19 1990 7.35 1991 7.52 1992 7.69 1993 7.86 1994 8.03 1995 8.21 1996 8.38 1997 8.55 1998 8.73 1999 8.9 2000 9.07 2001 9.24 2002 9.42 2003 9.59 2004 9.76 2005 9.93 2006 10.1 2007 10.27 2008 10.44 2009 10.61 2010 10.78 2011 10.95 2012 11.12 2013 11.28 2014 11.45 2015 11.61