Year	Years
1970	3.27
1971	3.34
1972	3.42
1973	3.5
1974	3.58
1975	3.66
1976	3.75
1977	3.83
1978	3.92
1979	4.01
1980	4.09
1981	4.19
1982	4.28
1983	4.37
1984	4.47
1985	4.56
1986	4.66
1987	4.76
1988	4.86
1989	4.96
1990	5.06
1991	5.17
1992	5.27
1993	5.38
1994	5.49
1995	5.6
1996	5.71
1997	5.82
1998	5.93
1999	6.05
2000	6.16
2001	6.28
2002	6.4
2003	6.52
2004	6.64
2005	6.76
2006	6.89
2007	7.02
2008	7.14
2009	7.27
2010	7.4
2011	7.53
2012	7.67
2013	7.8
2014	7.93
2015	8.07