Year	Years
1970	2.34
1971	2.4
1972	2.46
1973	2.52
1974	2.59
1975	2.65
1976	2.72
1977	2.79
1978	2.86
1979	2.93
1980	3
1981	3.07
1982	3.14
1983	3.21
1984	3.29
1985	3.36
1986	3.43
1987	3.51
1988	3.59
1989	3.67
1990	3.75
1991	3.83
1992	3.91
1993	4
1994	4.09
1995	4.18
1996	4.29
1997	4.4
1998	4.52
1999	4.64
2000	4.76
2001	4.89
2002	5.01
2003	5.14
2004	5.27
2005	5.4
2006	5.54
2007	5.68
2008	5.82
2009	5.96
2010	6.1
2011	6.25
2012	6.39
2013	6.54
2014	6.69
2015	6.85