Year	Years
1970	2.8
1971	2.87
1972	2.94
1973	3.01
1974	3.09
1975	3.16
1976	3.24
1977	3.32
1978	3.4
1979	3.48
1980	3.57
1981	3.65
1982	3.74
1983	3.83
1984	3.92
1985	4.01
1986	4.11
1987	4.2
1988	4.3
1989	4.4
1990	4.5
1991	4.6
1992	4.7
1993	4.81
1994	4.92
1995	5.03
1996	5.14
1997	5.25
1998	5.37
1999	5.49
2000	5.61
2001	5.73
2002	5.85
2003	5.97
2004	6.09
2005	6.22
2006	6.34
2007	6.47
2008	6.6
2009	6.73
2010	6.85
2011	6.98
2012	7.11
2013	7.24
2014	7.38
2015	7.51