Year	Years
1970	3.23
1971	3.3
1972	3.38
1973	3.46
1974	3.54
1975	3.62
1976	3.7
1977	3.78
1978	3.87
1979	3.95
1980	4.04
1981	4.13
1982	4.21
1983	4.3
1984	4.39
1985	4.49
1986	4.58
1987	4.67
1988	4.77
1989	4.86
1990	4.95
1991	5.05
1992	5.15
1993	5.24
1994	5.34
1995	5.44
1996	5.53
1997	5.63
1998	5.72
1999	5.82
2000	5.93
2001	6.03
2002	6.13
2003	6.24
2004	6.35
2005	6.46
2006	6.57
2007	6.69
2008	6.8
2009	6.92
2010	7.04
2011	7.16
2012	7.29
2013	7.41
2014	7.54
2015	7.67