Year	Years
1970	2.58
1971	2.66
1972	2.74
1973	2.83
1974	2.91
1975	3
1976	3.09
1977	3.19
1978	3.28
1979	3.38
1980	3.48
1981	3.59
1982	3.69
1983	3.8
1984	3.91
1985	4.03
1986	4.14
1987	4.26
1988	4.38
1989	4.5
1990	4.63
1991	4.75
1992	4.88
1993	5.01
1994	5.14
1995	5.28
1996	5.41
1997	5.55
1998	5.69
1999	5.83
2000	5.97
2001	6.11
2002	6.26
2003	6.4
2004	6.55
2005	6.7
2006	6.85
2007	7
2008	7.15
2009	7.3
2010	7.45
2011	7.61
2012	7.76
2013	7.92
2014	8.07
2015	8.23