Year	Years
1970	2.59
1971	2.67
1972	2.74
1973	2.82
1974	2.9
1975	2.98
1976	3.06
1977	3.14
1978	3.23
1979	3.32
1980	3.41
1981	3.51
1982	3.6
1983	3.7
1984	3.8
1985	3.9
1986	4.01
1987	4.11
1988	4.22
1989	4.33
1990	4.44
1991	4.56
1992	4.67
1993	4.79
1994	4.91
1995	5.03
1996	5.14
1997	5.25
1998	5.36
1999	5.48
2000	5.59
2001	5.71
2002	5.83
2003	5.94
2004	6.06
2005	6.18
2006	6.3
2007	6.42
2008	6.54
2009	6.67
2010	6.79
2011	6.91
2012	7.04
2013	7.16
2014	7.29
2015	7.42