Year	Years
1970	2.26
1971	2.33
1972	2.4
1973	2.47
1974	2.55
1975	2.63
1976	2.7
1977	2.78
1978	2.87
1979	2.95
1980	3.04
1981	3.12
1982	3.21
1983	3.31
1984	3.4
1985	3.5
1986	3.59
1987	3.7
1988	3.8
1989	3.9
1990	4.01
1991	4.12
1992	4.23
1993	4.34
1994	4.46
1995	4.58
1996	4.7
1997	4.82
1998	4.94
1999	5.07
2000	5.2
2001	5.33
2002	5.46
2003	5.6
2004	5.73
2005	5.87
2006	6.01
2007	6.16
2008	6.3
2009	6.45
2010	6.6
2011	6.74
2012	6.9
2013	7.05
2014	7.2
2015	7.36