Year	Years
1970	2.32
1971	2.38
1972	2.44
1973	2.5
1974	2.56
1975	2.62
1976	2.69
1977	2.75
1978	2.82
1979	2.89
1980	2.96
1981	3.03
1982	3.1
1983	3.18
1984	3.25
1985	3.33
1986	3.41
1987	3.49
1988	3.57
1989	3.65
1990	3.74
1991	3.83
1992	3.91
1993	4
1994	4.1
1995	4.19
1996	4.29
1997	4.39
1998	4.49
1999	4.59
2000	4.7
2001	4.81
2002	4.91
2003	5.02
2004	5.13
2005	5.24
2006	5.36
2007	5.47
2008	5.59
2009	5.71
2010	5.82
2011	5.94
2012	6.07
2013	6.19
2014	6.31
2015	6.44