Year	Years
1970	4.09
1971	4.18
1972	4.26
1973	4.34
1974	4.42
1975	4.5
1976	4.58
1977	4.66
1978	4.74
1979	4.81
1980	4.89
1981	4.96
1982	5.04
1983	5.11
1984	5.17
1985	5.24
1986	5.3
1987	5.36
1988	5.42
1989	5.47
1990	5.52
1991	5.57
1992	5.61
1993	5.66
1994	5.7
1995	5.74
1996	5.85
1997	5.97
1998	6.09
1999	6.21
2000	6.34
2001	6.46
2002	6.59
2003	6.73
2004	6.86
2005	7
2006	7.14
2007	7.28
2008	7.43
2009	7.58
2010	7.72
2011	7.87
2012	8.03
2013	8.18
2014	8.33
2015	8.49