Year	Years
1970	2.4
1971	2.46
1972	2.53
1973	2.59
1974	2.65
1975	2.72
1976	2.79
1977	2.86
1978	2.93
1979	3
1980	3.07
1981	3.15
1982	3.23
1983	3.31
1984	3.39
1985	3.47
1986	3.55
1987	3.64
1988	3.73
1989	3.81
1990	3.91
1991	4
1992	4.09
1993	4.19
1994	4.29
1995	4.39
1996	4.49
1997	4.6
1998	4.71
1999	4.81
2000	4.92
2001	5.03
2002	5.14
2003	5.25
2004	5.37
2005	5.48
2006	5.6
2007	5.72
2008	5.83
2009	5.95
2010	6.08
2011	6.2
2012	6.32
2013	6.45
2014	6.57
2015	6.7