Year	Years
1970	2.89
1971	2.96
1972	3.04
1973	3.11
1974	3.19
1975	3.27
1976	3.36
1977	3.44
1978	3.53
1979	3.62
1980	3.71
1981	3.81
1982	3.9
1983	4
1984	4.1
1985	4.2
1986	4.31
1987	4.42
1988	4.53
1989	4.64
1990	4.76
1991	4.88
1992	5
1993	5.12
1994	5.25
1995	5.37
1996	5.5
1997	5.63
1998	5.76
1999	5.89
2000	6.02
2001	6.16
2002	6.29
2003	6.42
2004	6.56
2005	6.69
2006	6.83
2007	6.97
2008	7.11
2009	7.24
2010	7.38
2011	7.52
2012	7.66
2013	7.8
2014	7.94
2015	8.08