Year	Years
1970	4.06
1971	4.14
1972	4.23
1973	4.31
1974	4.39
1975	4.47
1976	4.56
1977	4.64
1978	4.73
1979	4.81
1980	4.9
1981	4.98
1982	5.07
1983	5.15
1984	5.24
1985	5.32
1986	5.41
1987	5.5
1988	5.58
1989	5.67
1990	5.75
1991	5.84
1992	5.92
1993	6.01
1994	6.09
1995	6.18
1996	6.29
1997	6.4
1998	6.51
1999	6.63
2000	6.74
2001	6.86
2002	6.98
2003	7.11
2004	7.23
2005	7.36
2006	7.48
2007	7.61
2008	7.74
2009	7.87
2010	8.01
2011	8.14
2012	8.27
2013	8.41
2014	8.54
2015	8.68