Year	Years
1970	2.89
1971	2.96
1972	3.03
1973	3.1
1974	3.18
1975	3.26
1976	3.33
1977	3.41
1978	3.5
1979	3.58
1980	3.66
1981	3.75
1982	3.84
1983	3.93
1984	4.02
1985	4.11
1986	4.21
1987	4.31
1988	4.41
1989	4.51
1990	4.61
1991	4.71
1992	4.82
1993	4.93
1994	5.04
1995	5.15
1996	5.27
1997	5.39
1998	5.51
1999	5.63
2000	5.75
2001	5.88
2002	6
2003	6.13
2004	6.26
2005	6.39
2006	6.51
2007	6.65
2008	6.78
2009	6.91
2010	7.04
2011	7.17
2012	7.31
2013	7.44
2014	7.58
2015	7.71