Year	Years
1970	2.91
1971	2.98
1972	3.05
1973	3.12
1974	3.2
1975	3.27
1976	3.35
1977	3.43
1978	3.51
1979	3.59
1980	3.68
1981	3.76
1982	3.85
1983	3.94
1984	4.03
1985	4.12
1986	4.21
1987	4.3
1988	4.4
1989	4.5
1990	4.59
1991	4.69
1992	4.8
1993	4.9
1994	5
1995	5.11
1996	5.22
1997	5.33
1998	5.44
1999	5.55
2000	5.66
2001	5.77
2002	5.89
2003	6.01
2004	6.13
2005	6.25
2006	6.37
2007	6.49
2008	6.61
2009	6.74
2010	6.86
2011	6.99
2012	7.12
2013	7.25
2014	7.38
2015	7.51