Year	Years
1970	3.12
1971	3.2
1972	3.29
1973	3.37
1974	3.46
1975	3.55
1976	3.64
1977	3.73
1978	3.82
1979	3.91
1980	4.01
1981	4.1
1982	4.2
1983	4.29
1984	4.39
1985	4.48
1986	4.58
1987	4.67
1988	4.77
1989	4.86
1990	4.95
1991	5.05
1992	5.14
1993	5.23
1994	5.32
1995	5.41
1996	5.53
1997	5.65
1998	5.78
1999	5.9
2000	6.04
2001	6.17
2002	6.31
2003	6.45
2004	6.59
2005	6.74
2006	6.88
2007	7.03
2008	7.18
2009	7.34
2010	7.49
2011	7.64
2012	7.8
2013	7.96
2014	8.12
2015	8.28