Year	Years
1970	2.38
1971	2.45
1972	2.53
1973	2.6
1974	2.68
1975	2.76
1976	2.84
1977	2.93
1978	3.02
1979	3.1
1980	3.19
1981	3.29
1982	3.38
1983	3.48
1984	3.58
1985	3.68
1986	3.78
1987	3.89
1988	4
1989	4.11
1990	4.22
1991	4.34
1992	4.45
1993	4.57
1994	4.7
1995	4.82
1996	4.95
1997	5.08
1998	5.21
1999	5.34
2000	5.47
2001	5.61
2002	5.75
2003	5.89
2004	6.03
2005	6.17
2006	6.32
2007	6.46
2008	6.61
2009	6.76
2010	6.92
2011	7.07
2012	7.23
2013	7.38
2014	7.54
2015	7.7