Year	Years
1970	2.26
1971	2.32
1972	2.38
1973	2.44
1974	2.5
1975	2.57
1976	2.64
1977	2.7
1978	2.77
1979	2.85
1980	2.92
1981	2.99
1982	3.07
1983	3.15
1984	3.23
1985	3.31
1986	3.39
1987	3.48
1988	3.56
1989	3.65
1990	3.74
1991	3.83
1992	3.92
1993	4.02
1994	4.11
1995	4.21
1996	4.31
1997	4.41
1998	4.52
1999	4.62
2000	4.73
2001	4.83
2002	4.94
2003	5.05
2004	5.16
2005	5.27
2006	5.38
2007	5.5
2008	5.61
2009	5.72
2010	5.84
2011	5.95
2012	6.07
2013	6.19
2014	6.3
2015	6.42