Year	Years
1970	2.25
1971	2.31
1972	2.37
1973	2.43
1974	2.49
1975	2.55
1976	2.61
1977	2.67
1978	2.73
1979	2.8
1980	2.87
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	3.68
1992	3.76
1993	3.84
1994	3.92
1995	4
1996	4.09
1997	4.17
1998	4.26
1999	4.34
2000	4.43
2001	4.52
2002	4.61
2003	4.71
2004	4.8
2005	4.9
2006	5
2007	5.1
2008	5.2
2009	5.3
2010	5.41
2011	5.52
2012	5.63
2013	5.74
2014	5.85
2015	5.97