Year	Years
1970	2.29
1971	2.36
1972	2.44
1973	2.52
1974	2.6
1975	2.68
1976	2.77
1977	2.85
1978	2.94
1979	3.04
1980	3.13
1981	3.23
1982	3.33
1983	3.43
1984	3.54
1985	3.64
1986	3.75
1987	3.87
1988	3.98
1989	4.1
1990	4.22
1991	4.34
1992	4.47
1993	4.59
1994	4.72
1995	4.85
1996	4.98
1997	5.1
1998	5.23
1999	5.36
2000	5.49
2001	5.63
2002	5.76
2003	5.9
2004	6.04
2005	6.18
2006	6.33
2007	6.48
2008	6.63
2009	6.78
2010	6.93
2011	7.09
2012	7.25
2013	7.41
2014	7.57
2015	7.73