Year	Years
1970	2.58
1971	2.66
1972	2.74
1973	2.82
1974	2.9
1975	2.98
1976	3.07
1977	3.16
1978	3.25
1979	3.34
1980	3.43
1981	3.53
1982	3.63
1983	3.73
1984	3.83
1985	3.93
1986	4.04
1987	4.14
1988	4.25
1989	4.36
1990	4.48
1991	4.59
1992	4.71
1993	4.83
1994	4.94
1995	5.07
1996	5.19
1997	5.31
1998	5.44
1999	5.57
2000	5.7
2001	5.84
2002	5.97
2003	6.11
2004	6.25
2005	6.4
2006	6.54
2007	6.69
2008	6.84
2009	6.99
2010	7.14
2011	7.29
2012	7.45
2013	7.6
2014	7.76
2015	7.92