Year	Years
1970	2.4
1971	2.46
1972	2.52
1973	2.59
1974	2.65
1975	2.72
1976	2.79
1977	2.86
1978	2.93
1979	3
1980	3.08
1981	3.15
1982	3.23
1983	3.31
1984	3.39
1985	3.47
1986	3.56
1987	3.64
1988	3.73
1989	3.82
1990	3.91
1991	4
1992	4.09
1993	4.19
1994	4.28
1995	4.38
1996	4.48
1997	4.57
1998	4.67
1999	4.77
2000	4.87
2001	4.98
2002	5.08
2003	5.19
2004	5.29
2005	5.4
2006	5.51
2007	5.62
2008	5.73
2009	5.84
2010	5.95
2011	6.06
2012	6.18
2013	6.29
2014	6.41
2015	6.53