Year	Years
1970	2.25
1971	2.31
1972	2.36
1973	2.42
1974	2.48
1975	2.54
1976	2.6
1977	2.67
1978	2.73
1979	2.8
1980	2.86
1981	2.93
1982	3
1983	3.07
1984	3.14
1985	3.22
1986	3.29
1987	3.37
1988	3.44
1989	3.52
1990	3.6
1991	3.69
1992	3.77
1993	3.85
1994	3.94
1995	4.03
1996	4.12
1997	4.21
1998	4.3
1999	4.39
2000	4.49
2001	4.59
2002	4.69
2003	4.79
2004	4.89
2005	5
2006	5.1
2007	5.21
2008	5.32
2009	5.43
2010	5.55
2011	5.66
2012	5.78
2013	5.9
2014	6.02
2015	6.14