Year	Years
1970	2.36
1971	2.43
1972	2.51
1973	2.58
1974	2.66
1975	2.74
1976	2.82
1977	2.9
1978	2.99
1979	3.08
1980	3.17
1981	3.26
1982	3.35
1983	3.45
1984	3.55
1985	3.65
1986	3.75
1987	3.85
1988	3.96
1989	4.07
1990	4.18
1991	4.29
1992	4.41
1993	4.53
1994	4.65
1995	4.77
1996	4.9
1997	5.03
1998	5.16
1999	5.3
2000	5.43
2001	5.57
2002	5.71
2003	5.85
2004	6
2005	6.14
2006	6.29
2007	6.44
2008	6.59
2009	6.74
2010	6.89
2011	7.04
2012	7.2
2013	7.35
2014	7.51
2015	7.67