Year	Years
1970	3.36
1971	3.44
1972	3.52
1973	3.61
1974	3.69
1975	3.78
1976	3.87
1977	3.96
1978	4.05
1979	4.14
1980	4.23
1981	4.33
1982	4.43
1983	4.53
1984	4.63
1985	4.73
1986	4.83
1987	4.94
1988	5.04
1989	5.15
1990	5.26
1991	5.37
1992	5.48
1993	5.59
1994	5.71
1995	5.82
1996	5.94
1997	6.06
1998	6.18
1999	6.3
2000	6.42
2001	6.54
2002	6.67
2003	6.79
2004	6.92
2005	7.04
2006	7.17
2007	7.3
2008	7.42
2009	7.55
2010	7.68
2011	7.81
2012	7.95
2013	8.08
2014	8.21
2015	8.35