Year	Years
1970	2.52
1971	2.59
1972	2.65
1973	2.72
1974	2.79
1975	2.86
1976	2.93
1977	3.01
1978	3.08
1979	3.16
1980	3.24
1981	3.32
1982	3.4
1983	3.48
1984	3.57
1985	3.66
1986	3.74
1987	3.83
1988	3.93
1989	4.02
1990	4.11
1991	4.21
1992	4.31
1993	4.41
1994	4.51
1995	4.62
1996	4.72
1997	4.83
1998	4.93
1999	5.04
2000	5.15
2001	5.26
2002	5.37
2003	5.49
2004	5.6
2005	5.71
2006	5.83
2007	5.95
2008	6.06
2009	6.18
2010	6.3
2011	6.42
2012	6.54
2013	6.67
2014	6.79
2015	6.91