Year	Years
1970	2.35
1971	2.41
1972	2.47
1973	2.53
1974	2.59
1975	2.66
1976	2.72
1977	2.79
1978	2.86
1979	2.93
1980	3
1981	3.07
1982	3.14
1983	3.22
1984	3.3
1985	3.37
1986	3.45
1987	3.53
1988	3.62
1989	3.7
1990	3.78
1991	3.87
1992	3.96
1993	4.05
1994	4.14
1995	4.23
1996	4.32
1997	4.42
1998	4.51
1999	4.61
2000	4.71
2001	4.81
2002	4.91
2003	5.01
2004	5.12
2005	5.22
2006	5.33
2007	5.44
2008	5.55
2009	5.66
2010	5.77
2011	5.89
2012	6
2013	6.12
2014	6.24
2015	6.36