Year	Years
1970	2.31
1971	2.39
1972	2.47
1973	2.56
1974	2.65
1975	2.74
1976	2.83
1977	2.93
1978	3.03
1979	3.13
1980	3.24
1981	3.35
1982	3.46
1983	3.58
1984	3.7
1985	3.82
1986	3.94
1987	4.06
1988	4.19
1989	4.32
1990	4.45
1991	4.59
1992	4.72
1993	4.86
1994	4.99
1995	5.13
1996	5.25
1997	5.37
1998	5.49
1999	5.62
2000	5.74
2001	5.86
2002	5.99
2003	6.11
2004	6.24
2005	6.37
2006	6.5
2007	6.63
2008	6.76
2009	6.9
2010	7.04
2011	7.17
2012	7.32
2013	7.46
2014	7.6
2015	7.75