Year	Years
1970	2.92
1971	3
1972	3.09
1973	3.19
1974	3.28
1975	3.38
1976	3.47
1977	3.57
1978	3.68
1979	3.78
1980	3.89
1981	4
1982	4.11
1983	4.22
1984	4.34
1985	4.46
1986	4.58
1987	4.7
1988	4.83
1989	4.95
1990	5.08
1991	5.21
1992	5.35
1993	5.48
1994	5.62
1995	5.76
1996	5.9
1997	6.04
1998	6.18
1999	6.32
2000	6.47
2001	6.62
2002	6.77
2003	6.92
2004	7.07
2005	7.22
2006	7.38
2007	7.54
2008	7.69
2009	7.85
2010	8.01
2011	8.17
2012	8.33
2013	8.49
2014	8.66
2015	8.82