Year	Years
1970	2.38
1971	2.45
1972	2.52
1973	2.6
1974	2.67
1975	2.75
1976	2.83
1977	2.91
1978	3
1979	3.08
1980	3.17
1981	3.26
1982	3.35
1983	3.44
1984	3.54
1985	3.63
1986	3.73
1987	3.83
1988	3.94
1989	4.04
1990	4.15
1991	4.26
1992	4.37
1993	4.48
1994	4.59
1995	4.71
1996	4.83
1997	4.95
1998	5.08
1999	5.2
2000	5.33
2001	5.47
2002	5.6
2003	5.74
2004	5.87
2005	6.02
2006	6.16
2007	6.3
2008	6.45
2009	6.6
2010	6.75
2011	6.9
2012	7.05
2013	7.21
2014	7.36
2015	7.52