Year	Years
1970	3.38
1971	3.45
1972	3.53
1973	3.6
1974	3.68
1975	3.75
1976	3.83
1977	3.91
1978	3.98
1979	4.06
1980	4.14
1981	4.22
1982	4.31
1983	4.39
1984	4.47
1985	4.56
1986	4.64
1987	4.73
1988	4.82
1989	4.91
1990	5
1991	5.09
1992	5.19
1993	5.29
1994	5.39
1995	5.49
1996	5.6
1997	5.71
1998	5.83
1999	5.94
2000	6.06
2001	6.17
2002	6.29
2003	6.41
2004	6.53
2005	6.65
2006	6.77
2007	6.89
2008	7.01
2009	7.14
2010	7.26
2011	7.39
2012	7.51
2013	7.64
2014	7.77
2015	7.9