Year	Years
1970	2.26
1971	2.32
1972	2.39
1973	2.47
1974	2.54
1975	2.62
1976	2.7
1977	2.78
1978	2.86
1979	2.94
1980	3.03
1981	3.11
1982	3.2
1983	3.3
1984	3.39
1985	3.49
1986	3.58
1987	3.68
1988	3.79
1989	3.89
1990	4
1991	4.1
1992	4.21
1993	4.33
1994	4.44
1995	4.56
1996	4.67
1997	4.79
1998	4.91
1999	5.04
2000	5.16
2001	5.29
2002	5.42
2003	5.56
2004	5.69
2005	5.83
2006	5.96
2007	6.1
2008	6.25
2009	6.39
2010	6.54
2011	6.68
2012	6.83
2013	6.98
2014	7.14
2015	7.29