Year	Years
1970	2.35
1971	2.42
1972	2.5
1973	2.57
1974	2.65
1975	2.73
1976	2.82
1977	2.91
1978	3
1979	3.09
1980	3.19
1981	3.29
1982	3.39
1983	3.5
1984	3.61
1985	3.73
1986	3.85
1987	3.97
1988	4.1
1989	4.23
1990	4.36
1991	4.5
1992	4.65
1993	4.8
1994	4.95
1995	5.1
1996	5.24
1997	5.37
1998	5.5
1999	5.64
2000	5.77
2001	5.9
2002	6.04
2003	6.17
2004	6.31
2005	6.44
2006	6.58
2007	6.71
2008	6.85
2009	6.98
2010	7.12
2011	7.25
2012	7.39
2013	7.52
2014	7.66
2015	7.8