Year	Years
1970	2.41
1971	2.48
1972	2.55
1973	2.62
1974	2.69
1975	2.77
1976	2.84
1977	2.92
1978	3
1979	3.09
1980	3.17
1981	3.26
1982	3.35
1983	3.44
1984	3.53
1985	3.63
1986	3.73
1987	3.83
1988	3.94
1989	4.04
1990	4.15
1991	4.26
1992	4.38
1993	4.49
1994	4.61
1995	4.73
1996	4.86
1997	4.98
1998	5.11
1999	5.24
2000	5.37
2001	5.5
2002	5.63
2003	5.76
2004	5.9
2005	6.03
2006	6.16
2007	6.29
2008	6.42
2009	6.55
2010	6.68
2011	6.81
2012	6.93
2013	7.06
2014	7.18
2015	7.31