Year	Years
1970	4.03
1971	4.12
1972	4.23
1973	4.33
1974	4.43
1975	4.54
1976	4.65
1977	4.76
1978	4.88
1979	4.99
1980	5.11
1981	5.23
1982	5.36
1983	5.48
1984	5.61
1985	5.74
1986	5.88
1987	6.01
1988	6.15
1989	6.29
1990	6.43
1991	6.58
1992	6.73
1993	6.88
1994	7.03
1995	7.18
1996	7.32
1997	7.45
1998	7.59
1999	7.72
2000	7.85
2001	7.99
2002	8.12
2003	8.26
2004	8.39
2005	8.53
2006	8.67
2007	8.8
2008	8.94
2009	9.08
2010	9.22
2011	9.35
2012	9.49
2013	9.63
2014	9.77
2015	9.91