Year	Years
1970	3.18
1971	3.26
1972	3.34
1973	3.41
1974	3.49
1975	3.57
1976	3.66
1977	3.74
1978	3.83
1979	3.91
1980	4
1981	4.09
1982	4.18
1983	4.27
1984	4.37
1985	4.46
1986	4.56
1987	4.66
1988	4.76
1989	4.86
1990	4.96
1991	5.07
1992	5.18
1993	5.28
1994	5.39
1995	5.5
1996	5.62
1997	5.73
1998	5.85
1999	5.96
2000	6.08
2001	6.2
2002	6.32
2003	6.44
2004	6.57
2005	6.69
2006	6.82
2007	6.94
2008	7.07
2009	7.2
2010	7.33
2011	7.46
2012	7.59
2013	7.73
2014	7.86
2015	7.99