Year	Years
1970	4.27
1971	4.38
1972	4.49
1973	4.6
1974	4.72
1975	4.84
1976	4.96
1977	5.08
1978	5.2
1979	5.33
1980	5.46
1981	5.59
1982	5.73
1983	5.87
1984	6.01
1985	6.15
1986	6.29
1987	6.44
1988	6.59
1989	6.74
1990	6.89
1991	7.04
1992	7.19
1993	7.34
1994	7.5
1995	7.65
1996	7.78
1997	7.92
1998	8.06
1999	8.2
2000	8.33
2001	8.47
2002	8.61
2003	8.75
2004	8.88
2005	9.02
2006	9.16
2007	9.3
2008	9.43
2009	9.57
2010	9.71
2011	9.85
2012	9.98
2013	10.12
2014	10.26
2015	10.39