Year	Years
1970	3.11
1971	3.19
1972	3.27
1973	3.35
1974	3.43
1975	3.52
1976	3.6
1977	3.69
1978	3.77
1979	3.86
1980	3.95
1981	4.04
1982	4.13
1983	4.22
1984	4.32
1985	4.41
1986	4.51
1987	4.6
1988	4.7
1989	4.8
1990	4.9
1991	5
1992	5.11
1993	5.22
1994	5.33
1995	5.45
1996	5.58
1997	5.71
1998	5.84
1999	5.98
2000	6.12
2001	6.26
2002	6.4
2003	6.54
2004	6.68
2005	6.83
2006	6.97
2007	7.12
2008	7.27
2009	7.42
2010	7.57
2011	7.73
2012	7.88
2013	8.04
2014	8.19
2015	8.35