Year	Years
1970	2.93
1971	3.01
1972	3.09
1973	3.17
1974	3.26
1975	3.34
1976	3.43
1977	3.52
1978	3.62
1979	3.71
1980	3.81
1981	3.91
1982	4.02
1983	4.12
1984	4.23
1985	4.34
1986	4.45
1987	4.57
1988	4.69
1989	4.81
1990	4.93
1991	5.06
1992	5.18
1993	5.32
1994	5.45
1995	5.59
1996	5.71
1997	5.83
1998	5.96
1999	6.09
2000	6.21
2001	6.34
2002	6.47
2003	6.6
2004	6.73
2005	6.86
2006	6.99
2007	7.12
2008	7.26
2009	7.39
2010	7.52
2011	7.66
2012	7.79
2013	7.93
2014	8.06
2015	8.2