Year	Years
1970	2.81
1971	2.88
1972	2.96
1973	3.05
1974	3.13
1975	3.22
1976	3.31
1977	3.4
1978	3.5
1979	3.6
1980	3.7
1981	3.8
1982	3.91
1983	4.02
1984	4.13
1985	4.24
1986	4.36
1987	4.49
1988	4.61
1989	4.74
1990	4.87
1991	5.01
1992	5.14
1993	5.29
1994	5.43
1995	5.58
1996	5.71
1997	5.85
1998	5.98
1999	6.12
2000	6.25
2001	6.38
2002	6.52
2003	6.65
2004	6.79
2005	6.92
2006	7.05
2007	7.19
2008	7.32
2009	7.46
2010	7.59
2011	7.73
2012	7.86
2013	8
2014	8.13
2015	8.27