Year	Years
1970	2.86
1971	2.94
1972	3.02
1973	3.11
1974	3.2
1975	3.29
1976	3.39
1977	3.48
1978	3.58
1979	3.69
1980	3.79
1981	3.9
1982	4.01
1983	4.13
1984	4.24
1985	4.36
1986	4.49
1987	4.62
1988	4.75
1989	4.88
1990	5.01
1991	5.15
1992	5.29
1993	5.44
1994	5.58
1995	5.73
1996	5.87
1997	6
1998	6.14
1999	6.27
2000	6.41
2001	6.54
2002	6.68
2003	6.81
2004	6.94
2005	7.08
2006	7.21
2007	7.34
2008	7.47
2009	7.6
2010	7.73
2011	7.86
2012	7.99
2013	8.12
2014	8.25
2015	8.38