Year	Years
1970	3.28
1971	3.36
1972	3.44
1973	3.52
1974	3.61
1975	3.69
1976	3.78
1977	3.87
1978	3.96
1979	4.05
1980	4.15
1981	4.24
1982	4.34
1983	4.44
1984	4.54
1985	4.64
1986	4.75
1987	4.85
1988	4.96
1989	5.07
1990	5.18
1991	5.29
1992	5.41
1993	5.53
1994	5.64
1995	5.76
1996	5.89
1997	6.01
1998	6.13
1999	6.26
2000	6.38
2001	6.51
2002	6.64
2003	6.77
2004	6.9
2005	7.03
2006	7.16
2007	7.29
2008	7.43
2009	7.56
2010	7.7
2011	7.83
2012	7.97
2013	8.11
2014	8.25
2015	8.39