Year	Years
1970	2.41
1971	2.47
1972	2.53
1973	2.59
1974	2.66
1975	2.72
1976	2.79
1977	2.86
1978	2.93
1979	3
1980	3.07
1981	3.15
1982	3.22
1983	3.3
1984	3.38
1985	3.46
1986	3.54
1987	3.62
1988	3.7
1989	3.79
1990	3.88
1991	3.97
1992	4.06
1993	4.15
1994	4.25
1995	4.34
1996	4.44
1997	4.53
1998	4.63
1999	4.73
2000	4.83
2001	4.94
2002	5.04
2003	5.15
2004	5.26
2005	5.36
2006	5.47
2007	5.59
2008	5.7
2009	5.81
2010	5.93
2011	6.05
2012	6.17
2013	6.29
2014	6.41
2015	6.53