Year	Years
1970	2.41
1971	2.47
1972	2.54
1973	2.6
1974	2.67
1975	2.73
1976	2.8
1977	2.87
1978	2.94
1979	3.02
1980	3.09
1981	3.17
1982	3.25
1983	3.33
1984	3.41
1985	3.49
1986	3.57
1987	3.66
1988	3.75
1989	3.84
1990	3.93
1991	4.02
1992	4.12
1993	4.22
1994	4.32
1995	4.42
1996	4.52
1997	4.63
1998	4.73
1999	4.84
2000	4.95
2001	5.06
2002	5.17
2003	5.28
2004	5.4
2005	5.51
2006	5.63
2007	5.74
2008	5.86
2009	5.98
2010	6.1
2011	6.22
2012	6.34
2013	6.46
2014	6.59
2015	6.71