Year	Years
1970	3.34
1971	3.42
1972	3.5
1973	3.59
1974	3.67
1975	3.76
1976	3.84
1977	3.93
1978	4.02
1979	4.12
1980	4.21
1981	4.31
1982	4.4
1983	4.5
1984	4.6
1985	4.7
1986	4.8
1987	4.91
1988	5.01
1989	5.12
1990	5.23
1991	5.34
1992	5.45
1993	5.56
1994	5.67
1995	5.78
1996	5.9
1997	6.02
1998	6.13
1999	6.25
2000	6.38
2001	6.5
2002	6.62
2003	6.75
2004	6.87
2005	7
2006	7.13
2007	7.26
2008	7.39
2009	7.52
2010	7.65
2011	7.78
2012	7.92
2013	8.05
2014	8.18
2015	8.32