Year	Years
1970	2.35
1971	2.42
1972	2.5
1973	2.58
1974	2.66
1975	2.75
1976	2.83
1977	2.92
1978	3
1979	3.09
1980	3.19
1981	3.28
1982	3.37
1983	3.47
1984	3.56
1985	3.66
1986	3.76
1987	3.86
1988	3.96
1989	4.06
1990	4.16
1991	4.26
1992	4.36
1993	4.47
1994	4.57
1995	4.67
1996	4.78
1997	4.88
1998	4.99
1999	5.1
2000	5.21
2001	5.32
2002	5.44
2003	5.55
2004	5.67
2005	5.79
2006	5.92
2007	6.04
2008	6.17
2009	6.3
2010	6.43
2011	6.57
2012	6.7
2013	6.84
2014	6.98
2015	7.12