Year	Years
1970	2.67
1971	2.74
1972	2.82
1973	2.9
1974	2.98
1975	3.06
1976	3.15
1977	3.23
1978	3.32
1979	3.41
1980	3.5
1981	3.59
1982	3.68
1983	3.78
1984	3.87
1985	3.97
1986	4.07
1987	4.17
1988	4.27
1989	4.37
1990	4.48
1991	4.59
1992	4.69
1993	4.8
1994	4.91
1995	5.03
1996	5.16
1997	5.29
1998	5.42
1999	5.55
2000	5.69
2001	5.83
2002	5.97
2003	6.12
2004	6.26
2005	6.41
2006	6.56
2007	6.71
2008	6.86
2009	7.01
2010	7.17
2011	7.32
2012	7.48
2013	7.64
2014	7.8
2015	7.96