Year	Years
1970	2.34
1971	2.42
1972	2.49
1973	2.57
1974	2.65
1975	2.73
1976	2.81
1977	2.9
1978	2.98
1979	3.07
1980	3.17
1981	3.26
1982	3.36
1983	3.46
1984	3.56
1985	3.66
1986	3.77
1987	3.88
1988	3.99
1989	4.1
1990	4.22
1991	4.34
1992	4.46
1993	4.58
1994	4.71
1995	4.84
1996	4.97
1997	5.1
1998	5.23
1999	5.36
2000	5.5
2001	5.63
2002	5.77
2003	5.91
2004	6.06
2005	6.2
2006	6.35
2007	6.5
2008	6.65
2009	6.8
2010	6.95
2011	7.11
2012	7.26
2013	7.42
2014	7.58
2015	7.74