Year	Years
1970	3.28
1971	3.36
1972	3.44
1973	3.52
1974	3.61
1975	3.69
1976	3.78
1977	3.86
1978	3.95
1979	4.04
1980	4.14
1981	4.23
1982	4.33
1983	4.42
1984	4.52
1985	4.62
1986	4.73
1987	4.83
1988	4.93
1989	5.04
1990	5.15
1991	5.26
1992	5.37
1993	5.49
1994	5.61
1995	5.72
1996	5.84
1997	5.97
1998	6.09
1999	6.21
2000	6.34
2001	6.47
2002	6.59
2003	6.72
2004	6.85
2005	6.98
2006	7.11
2007	7.25
2008	7.38
2009	7.51
2010	7.65
2011	7.78
2012	7.92
2013	8.06
2014	8.19
2015	8.33