Year	Years
1970	2.46
1971	2.53
1972	2.59
1973	2.66
1974	2.72
1975	2.79
1976	2.86
1977	2.93
1978	3
1979	3.07
1980	3.14
1981	3.2
1982	3.27
1983	3.34
1984	3.41
1985	3.48
1986	3.55
1987	3.62
1988	3.69
1989	3.76
1990	3.82
1991	3.89
1992	3.96
1993	4.03
1994	4.11
1995	4.19
1996	4.29
1997	4.39
1998	4.49
1999	4.59
2000	4.7
2001	4.81
2002	4.92
2003	5.03
2004	5.14
2005	5.26
2006	5.37
2007	5.49
2008	5.61
2009	5.73
2010	5.85
2011	5.98
2012	6.1
2013	6.23
2014	6.36
2015	6.49