Year	Years
1970	2.33
1971	2.39
1972	2.45
1973	2.52
1974	2.59
1975	2.66
1976	2.73
1977	2.8
1978	2.88
1979	2.95
1980	3.03
1981	3.11
1982	3.19
1983	3.28
1984	3.36
1985	3.45
1986	3.54
1987	3.63
1988	3.72
1989	3.81
1990	3.91
1991	4
1992	4.1
1993	4.2
1994	4.3
1995	4.4
1996	4.51
1997	4.61
1998	4.72
1999	4.82
2000	4.93
2001	5.03
2002	5.14
2003	5.25
2004	5.35
2005	5.46
2006	5.57
2007	5.67
2008	5.78
2009	5.89
2010	5.99
2011	6.1
2012	6.21
2013	6.32
2014	6.43
2015	6.54