Year	Years
1970	2.36
1971	2.44
1972	2.53
1973	2.61
1974	2.7
1975	2.8
1976	2.9
1977	3
1978	3.1
1979	3.21
1980	3.32
1981	3.44
1982	3.56
1983	3.68
1984	3.81
1985	3.95
1986	4.09
1987	4.23
1988	4.38
1989	4.54
1990	4.7
1991	4.87
1992	5.04
1993	5.22
1994	5.4
1995	5.59
1996	5.73
1997	5.87
1998	6.02
1999	6.17
2000	6.32
2001	6.47
2002	6.62
2003	6.77
2004	6.93
2005	7.08
2006	7.24
2007	7.4
2008	7.56
2009	7.72
2010	7.88
2011	8.05
2012	8.21
2013	8.38
2014	8.54
2015	8.71