Year	Years
1970	2.84
1971	2.91
1972	2.98
1973	3.05
1974	3.12
1975	3.2
1976	3.27
1977	3.35
1978	3.43
1979	3.51
1980	3.59
1981	3.67
1982	3.76
1983	3.84
1984	3.93
1985	4.02
1986	4.11
1987	4.2
1988	4.3
1989	4.39
1990	4.49
1991	4.59
1992	4.69
1993	4.79
1994	4.89
1995	4.99
1996	5.1
1997	5.21
1998	5.31
1999	5.42
2000	5.54
2001	5.65
2002	5.76
2003	5.88
2004	6
2005	6.11
2006	6.23
2007	6.36
2008	6.48
2009	6.6
2010	6.73
2011	6.85
2012	6.98
2013	7.11
2014	7.24
2015	7.37