Year	Years
1970	2.47
1971	2.55
1972	2.62
1973	2.7
1974	2.78
1975	2.86
1976	2.94
1977	3.03
1978	3.12
1979	3.21
1980	3.3
1981	3.39
1982	3.49
1983	3.59
1984	3.69
1985	3.79
1986	3.89
1987	4
1988	4.11
1989	4.22
1990	4.33
1991	4.44
1992	4.56
1993	4.68
1994	4.8
1995	4.93
1996	5.05
1997	5.18
1998	5.31
1999	5.44
2000	5.58
2001	5.71
2002	5.85
2003	5.99
2004	6.14
2005	6.28
2006	6.43
2007	6.58
2008	6.73
2009	6.88
2010	7.03
2011	7.19
2012	7.34
2013	7.5
2014	7.66
2015	7.82