Year	Years
1970	2.91
1971	2.98
1972	3.04
1973	3.11
1974	3.17
1975	3.24
1976	3.3
1977	3.37
1978	3.44
1979	3.51
1980	3.57
1981	3.64
1982	3.71
1983	3.78
1984	3.85
1985	3.92
1986	3.98
1987	4.05
1988	4.12
1989	4.18
1990	4.25
1991	4.31
1992	4.37
1993	4.43
1994	4.5
1995	4.56
1996	4.65
1997	4.74
1998	4.83
1999	4.93
2000	5.03
2001	5.13
2002	5.23
2003	5.33
2004	5.44
2005	5.55
2006	5.66
2007	5.77
2008	5.89
2009	6
2010	6.12
2011	6.24
2012	6.36
2013	6.48
2014	6.6
2015	6.72