Year	Years
1970	3.26
1971	3.34
1972	3.42
1973	3.5
1974	3.57
1975	3.65
1976	3.73
1977	3.82
1978	3.9
1979	3.98
1980	4.06
1981	4.14
1982	4.23
1983	4.31
1984	4.39
1985	4.48
1986	4.56
1987	4.65
1988	4.73
1989	4.82
1990	4.9
1991	4.99
1992	5.07
1993	5.16
1994	5.24
1995	5.32
1996	5.41
1997	5.5
1998	5.59
1999	5.69
2000	5.78
2001	5.88
2002	5.98
2003	6.08
2004	6.18
2005	6.28
2006	6.39
2007	6.5
2008	6.61
2009	6.72
2010	6.84
2011	6.95
2012	7.07
2013	7.19
2014	7.32
2015	7.44