Year	Years
1970	2.23
1971	2.28
1972	2.34
1973	2.4
1974	2.47
1975	2.53
1976	2.59
1977	2.66
1978	2.73
1979	2.8
1980	2.87
1981	2.94
1982	3.02
1983	3.1
1984	3.17
1985	3.25
1986	3.34
1987	3.42
1988	3.5
1989	3.59
1990	3.68
1991	3.77
1992	3.87
1993	3.96
1994	4.06
1995	4.16
1996	4.26
1997	4.36
1998	4.47
1999	4.57
2000	4.68
2001	4.79
2002	4.9
2003	5.01
2004	5.12
2005	5.23
2006	5.35
2007	5.46
2008	5.58
2009	5.69
2010	5.81
2011	5.93
2012	6.05
2013	6.17
2014	6.29
2015	6.41