Year	Years
1970	2.35
1971	2.43
1972	2.5
1973	2.58
1974	2.66
1975	2.74
1976	2.83
1977	2.91
1978	3
1979	3.1
1980	3.19
1981	3.29
1982	3.39
1983	3.49
1984	3.6
1985	3.71
1986	3.82
1987	3.93
1988	4.05
1989	4.17
1990	4.3
1991	4.43
1992	4.56
1993	4.7
1994	4.84
1995	4.99
1996	5.14
1997	5.29
1998	5.44
1999	5.6
2000	5.76
2001	5.91
2002	6.07
2003	6.23
2004	6.4
2005	6.56
2006	6.73
2007	6.89
2008	7.06
2009	7.23
2010	7.4
2011	7.57
2012	7.74
2013	7.91
2014	8.08
2015	8.25