Year	Years
1970	2.77
1971	2.85
1972	2.94
1973	3.02
1974	3.11
1975	3.2
1976	3.29
1977	3.39
1978	3.48
1979	3.58
1980	3.68
1981	3.78
1982	3.89
1983	3.99
1984	4.1
1985	4.21
1986	4.33
1987	4.44
1988	4.56
1989	4.68
1990	4.8
1991	4.93
1992	5.05
1993	5.18
1994	5.31
1995	5.44
1996	5.58
1997	5.72
1998	5.86
1999	6
2000	6.14
2001	6.28
2002	6.43
2003	6.58
2004	6.73
2005	6.88
2006	7.03
2007	7.19
2008	7.34
2009	7.5
2010	7.66
2011	7.82
2012	7.98
2013	8.14
2014	8.3
2015	8.47