Year	Years
1970	2.2
1971	2.26
1972	2.32
1973	2.38
1974	2.44
1975	2.5
1976	2.56
1977	2.63
1978	2.69
1979	2.76
1980	2.83
1981	2.9
1982	2.97
1983	3.04
1984	3.11
1985	3.19
1986	3.27
1987	3.34
1988	3.42
1989	3.51
1990	3.59
1991	3.67
1992	3.76
1993	3.84
1994	3.93
1995	4.02
1996	4.11
1997	4.2
1998	4.29
1999	4.39
2000	4.48
2001	4.58
2002	4.67
2003	4.77
2004	4.87
2005	4.98
2006	5.08
2007	5.19
2008	5.29
2009	5.4
2010	5.51
2011	5.62
2012	5.74
2013	5.85
2014	5.97
2015	6.09