Year	Years
1970	2.23
1971	2.29
1972	2.35
1973	2.41
1974	2.47
1975	2.53
1976	2.6
1977	2.67
1978	2.73
1979	2.8
1980	2.87
1981	2.95
1982	3.02
1983	3.1
1984	3.17
1985	3.25
1986	3.33
1987	3.42
1988	3.5
1989	3.59
1990	3.67
1991	3.76
1992	3.86
1993	3.95
1994	4.05
1995	4.14
1996	4.24
1997	4.35
1998	4.45
1999	4.55
2000	4.66
2001	4.77
2002	4.87
2003	4.98
2004	5.09
2005	5.2
2006	5.32
2007	5.43
2008	5.54
2009	5.66
2010	5.78
2011	5.89
2012	6.01
2013	6.13
2014	6.25
2015	6.38