Year	Years
1970	2.54
1971	2.61
1972	2.67
1973	2.74
1974	2.8
1975	2.87
1976	2.94
1977	3.01
1978	3.08
1979	3.15
1980	3.23
1981	3.3
1982	3.38
1983	3.46
1984	3.54
1985	3.62
1986	3.7
1987	3.78
1988	3.87
1989	3.95
1990	4.04
1991	4.13
1992	4.21
1993	4.3
1994	4.39
1995	4.48
1996	4.57
1997	4.67
1998	4.76
1999	4.85
2000	4.95
2001	5.05
2002	5.15
2003	5.25
2004	5.35
2005	5.45
2006	5.56
2007	5.67
2008	5.78
2009	5.89
2010	6
2011	6.11
2012	6.23
2013	6.35
2014	6.47
2015	6.59