Year	Years
1970	2.33
1971	2.39
1972	2.46
1973	2.52
1974	2.58
1975	2.65
1976	2.72
1977	2.79
1978	2.86
1979	2.93
1980	3.01
1981	3.08
1982	3.16
1983	3.24
1984	3.32
1985	3.41
1986	3.49
1987	3.58
1988	3.67
1989	3.76
1990	3.85
1991	3.95
1992	4.05
1993	4.15
1994	4.25
1995	4.35
1996	4.45
1997	4.55
1998	4.66
1999	4.76
2000	4.87
2001	4.97
2002	5.08
2003	5.19
2004	5.3
2005	5.41
2006	5.53
2007	5.64
2008	5.76
2009	5.87
2010	5.99
2011	6.11
2012	6.23
2013	6.35
2014	6.48
2015	6.6