Year	Years
1970	2.56
1971	2.62
1972	2.69
1973	2.75
1974	2.82
1975	2.89
1976	2.96
1977	3.03
1978	3.1
1979	3.18
1980	3.26
1981	3.33
1982	3.41
1983	3.5
1984	3.58
1985	3.66
1986	3.75
1987	3.84
1988	3.93
1989	4.02
1990	4.12
1991	4.21
1992	4.31
1993	4.41
1994	4.51
1995	4.62
1996	4.73
1997	4.84
1998	4.95
1999	5.07
2000	5.18
2001	5.3
2002	5.42
2003	5.54
2004	5.66
2005	5.78
2006	5.91
2007	6.03
2008	6.16
2009	6.29
2010	6.41
2011	6.54
2012	6.67
2013	6.81
2014	6.94
2015	7.07