Year	Years
1970	3.22
1971	3.31
1972	3.41
1973	3.51
1974	3.61
1975	3.71
1976	3.81
1977	3.92
1978	4.02
1979	4.12
1980	4.22
1981	4.33
1982	4.43
1983	4.53
1984	4.63
1985	4.73
1986	4.82
1987	4.92
1988	5.02
1989	5.11
1990	5.21
1991	5.3
1992	5.4
1993	5.49
1994	5.58
1995	5.67
1996	5.77
1997	5.87
1998	5.97
1999	6.07
2000	6.17
2001	6.28
2002	6.39
2003	6.5
2004	6.62
2005	6.74
2006	6.86
2007	6.98
2008	7.11
2009	7.24
2010	7.38
2011	7.51
2012	7.65
2013	7.79
2014	7.94
2015	8.08