Year	Years
1970	2.54
1971	2.61
1972	2.67
1973	2.74
1974	2.81
1975	2.87
1976	2.94
1977	3.02
1978	3.09
1979	3.16
1980	3.24
1981	3.32
1982	3.4
1983	3.48
1984	3.56
1985	3.64
1986	3.73
1987	3.82
1988	3.91
1989	4
1990	4.09
1991	4.18
1992	4.28
1993	4.38
1994	4.48
1995	4.58
1996	4.69
1997	4.8
1998	4.91
1999	5.02
2000	5.13
2001	5.24
2002	5.36
2003	5.47
2004	5.59
2005	5.71
2006	5.83
2007	5.95
2008	6.07
2009	6.19
2010	6.31
2011	6.44
2012	6.56
2013	6.69
2014	6.82
2015	6.94