Year	Years
1970	3.38
1971	3.46
1972	3.54
1973	3.62
1974	3.71
1975	3.79
1976	3.88
1977	3.97
1978	4.06
1979	4.15
1980	4.24
1981	4.34
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	6.55
2002	6.67
2003	6.8
2004	6.93
2005	7.05
2006	7.18
2007	7.31
2008	7.45
2009	7.58
2010	7.71
2011	7.85
2012	7.98
2013	8.12
2014	8.26
2015	8.39