Year Years 1970 5.46 1971 5.59 1972 5.73 1973 5.86 1974 6 1975 6.14 1976 6.28 1977 6.42 1978 6.57 1979 6.71 1980 6.86 1981 7 1982 7.15 1983 7.3 1984 7.45 1985 7.59 1986 7.74 1987 7.89 1988 8.04 1989 8.19 1990 8.35 1991 8.5 1992 8.65 1993 8.8 1994 8.95 1995 9.1 1996 9.26 1997 9.42 1998 9.58 1999 9.74 2000 9.9 2001 10.06 2002 10.22 2003 10.39 2004 10.55 2005 10.71 2006 10.87 2007 11.04 2008 11.2 2009 11.36 2010 11.52 2011 11.68 2012 11.84 2013 12 2014 12.16 2015 12.32