Year	Years
1970	2.34
1971	2.41
1972	2.48
1973	2.56
1974	2.64
1975	2.71
1976	2.8
1977	2.88
1978	2.96
1979	3.05
1980	3.14
1981	3.23
1982	3.32
1983	3.42
1984	3.51
1985	3.61
1986	3.71
1987	3.81
1988	3.92
1989	4.02
1990	4.13
1991	4.24
1992	4.35
1993	4.46
1994	4.58
1995	4.69
1996	4.81
1997	4.93
1998	5.06
1999	5.18
2000	5.31
2001	5.43
2002	5.56
2003	5.69
2004	5.83
2005	5.96
2006	6.1
2007	6.24
2008	6.38
2009	6.52
2010	6.66
2011	6.81
2012	6.95
2013	7.1
2014	7.25
2015	7.4