Year	Years
1970	3.78
1971	3.87
1972	3.96
1973	4.05
1974	4.14
1975	4.23
1976	4.32
1977	4.42
1978	4.52
1979	4.61
1980	4.71
1981	4.81
1982	4.91
1983	5.02
1984	5.12
1985	5.22
1986	5.33
1987	5.43
1988	5.54
1989	5.64
1990	5.75
1991	5.85
1992	5.96
1993	6.07
1994	6.17
1995	6.28
1996	6.38
1997	6.49
1998	6.6
1999	6.71
2000	6.82
2001	6.93
2002	7.05
2003	7.17
2004	7.28
2005	7.4
2006	7.52
2007	7.65
2008	7.77
2009	7.9
2010	8.02
2011	8.15
2012	8.28
2013	8.41
2014	8.54
2015	8.67