Year	Years
1970	3.07
1971	3.15
1972	3.22
1973	3.3
1974	3.37
1975	3.45
1976	3.53
1977	3.61
1978	3.69
1979	3.78
1980	3.86
1981	3.95
1982	4.04
1983	4.12
1984	4.22
1985	4.31
1986	4.4
1987	4.5
1988	4.59
1989	4.69
1990	4.79
1991	4.89
1992	4.99
1993	5.1
1994	5.2
1995	5.31
1996	5.42
1997	5.53
1998	5.65
1999	5.76
2000	5.88
2001	6
2002	6.12
2003	6.24
2004	6.36
2005	6.48
2006	6.61
2007	6.73
2008	6.86
2009	6.99
2010	7.12
2011	7.25
2012	7.38
2013	7.52
2014	7.65
2015	7.79