Year	Years
1970	2.37
1971	2.44
1972	2.51
1973	2.59
1974	2.66
1975	2.74
1976	2.82
1977	2.9
1978	2.99
1979	3.07
1980	3.16
1981	3.25
1982	3.34
1983	3.43
1984	3.53
1985	3.62
1986	3.72
1987	3.82
1988	3.93
1989	4.03
1990	4.14
1991	4.25
1992	4.36
1993	4.47
1994	4.59
1995	4.7
1996	4.82
1997	4.95
1998	5.07
1999	5.2
2000	5.33
2001	5.46
2002	5.59
2003	5.73
2004	5.87
2005	6.01
2006	6.15
2007	6.29
2008	6.44
2009	6.59
2010	6.74
2011	6.89
2012	7.04
2013	7.2
2014	7.36
2015	7.51