Year	Years
1970	2.41
1971	2.47
1972	2.53
1973	2.59
1974	2.66
1975	2.72
1976	2.79
1977	2.85
1978	2.92
1979	2.99
1980	3.06
1981	3.13
1982	3.21
1983	3.28
1984	3.36
1985	3.44
1986	3.51
1987	3.6
1988	3.68
1989	3.76
1990	3.85
1991	3.93
1992	4.02
1993	4.11
1994	4.21
1995	4.3
1996	4.4
1997	4.5
1998	4.6
1999	4.7
2000	4.8
2001	4.91
2002	5.02
2003	5.12
2004	5.24
2005	5.35
2006	5.46
2007	5.58
2008	5.69
2009	5.81
2010	5.93
2011	6.05
2012	6.18
2013	6.3
2014	6.43
2015	6.55