Year	Years
1970	2.52
1971	2.59
1972	2.66
1973	2.73
1974	2.81
1975	2.89
1976	2.97
1977	3.05
1978	3.14
1979	3.23
1980	3.32
1981	3.41
1982	3.51
1983	3.61
1984	3.71
1985	3.82
1986	3.93
1987	4.04
1988	4.15
1989	4.27
1990	4.39
1991	4.52
1992	4.65
1993	4.78
1994	4.91
1995	5.05
1996	5.17
1997	5.29
1998	5.42
1999	5.54
2000	5.66
2001	5.78
2002	5.91
2003	6.03
2004	6.16
2005	6.28
2006	6.4
2007	6.53
2008	6.66
2009	6.78
2010	6.91
2011	7.04
2012	7.16
2013	7.29
2014	7.42
2015	7.55