Year	Years
1970	2.36
1971	2.43
1972	2.5
1973	2.57
1974	2.64
1975	2.71
1976	2.79
1977	2.86
1978	2.94
1979	3.02
1980	3.1
1981	3.18
1982	3.26
1983	3.34
1984	3.43
1985	3.51
1986	3.6
1987	3.68
1988	3.77
1989	3.85
1990	3.94
1991	4.03
1992	4.12
1993	4.2
1994	4.29
1995	4.37
1996	4.46
1997	4.55
1998	4.64
1999	4.74
2000	4.84
2001	4.94
2002	5.05
2003	5.16
2004	5.27
2005	5.38
2006	5.5
2007	5.62
2008	5.74
2009	5.87
2010	6
2011	6.13
2012	6.26
2013	6.39
2014	6.53
2015	6.67