Year	Years
1970	4.52
1971	4.59
1972	4.67
1973	4.74
1974	4.82
1975	4.89
1976	4.97
1977	5.04
1978	5.11
1979	5.18
1980	5.25
1981	5.31
1982	5.38
1983	5.44
1984	5.5
1985	5.56
1986	5.62
1987	5.67
1988	5.72
1989	5.76
1990	5.81
1991	5.84
1992	5.88
1993	5.91
1994	5.94
1995	5.96
1996	6.04
1997	6.13
1998	6.21
1999	6.3
2000	6.4
2001	6.49
2002	6.59
2003	6.69
2004	6.8
2005	6.9
2006	7.01
2007	7.12
2008	7.23
2009	7.35
2010	7.46
2011	7.58
2012	7.7
2013	7.82
2014	7.95
2015	8.07