Year	Years
1970	2.79
1971	2.86
1972	2.93
1973	3
1974	3.08
1975	3.15
1976	3.23
1977	3.31
1978	3.39
1979	3.47
1980	3.56
1981	3.64
1982	3.73
1983	3.82
1984	3.91
1985	4
1986	4.1
1987	4.19
1988	4.29
1989	4.39
1990	4.49
1991	4.6
1992	4.7
1993	4.81
1994	4.92
1995	5.03
1996	5.14
1997	5.26
1998	5.38
1999	5.49
2000	5.61
2001	5.73
2002	5.86
2003	5.98
2004	6.1
2005	6.23
2006	6.35
2007	6.48
2008	6.61
2009	6.74
2010	6.86
2011	6.99
2012	7.12
2013	7.25
2014	7.39
2015	7.52