Year	Years
1970	2.93
1971	3.02
1972	3.1
1973	3.18
1974	3.27
1975	3.36
1976	3.45
1977	3.54
1978	3.64
1979	3.73
1980	3.83
1981	3.93
1982	4.04
1983	4.14
1984	4.25
1985	4.36
1986	4.47
1987	4.58
1988	4.7
1989	4.82
1990	4.94
1991	5.06
1992	5.19
1993	5.32
1994	5.45
1995	5.59
1996	5.73
1997	5.88
1998	6.03
1999	6.18
2000	6.33
2001	6.48
2002	6.64
2003	6.79
2004	6.95
2005	7.11
2006	7.27
2007	7.43
2008	7.6
2009	7.76
2010	7.93
2011	8.09
2012	8.26
2013	8.43
2014	8.59
2015	8.76