Year	Years
1970	2.66
1971	2.74
1972	2.82
1973	2.9
1974	2.99
1975	3.08
1976	3.17
1977	3.26
1978	3.36
1979	3.45
1980	3.55
1981	3.65
1982	3.76
1983	3.86
1984	3.97
1985	4.08
1986	4.19
1987	4.31
1988	4.42
1989	4.54
1990	4.66
1991	4.79
1992	4.91
1993	5.04
1994	5.17
1995	5.31
1996	5.44
1997	5.58
1998	5.71
1999	5.85
2000	5.99
2001	6.14
2002	6.28
2003	6.43
2004	6.58
2005	6.73
2006	6.88
2007	7.04
2008	7.19
2009	7.35
2010	7.51
2011	7.67
2012	7.83
2013	7.99
2014	8.15
2015	8.32