Year	Years
1970	4.38
1971	4.47
1972	4.57
1973	4.67
1974	4.76
1975	4.87
1976	4.97
1977	5.07
1978	5.18
1979	5.28
1980	5.39
1981	5.5
1982	5.61
1983	5.72
1984	5.83
1985	5.94
1986	6.06
1987	6.18
1988	6.29
1989	6.41
1990	6.53
1991	6.65
1992	6.78
1993	6.9
1994	7.03
1995	7.15
1996	7.29
1997	7.42
1998	7.55
1999	7.69
2000	7.82
2001	7.96
2002	8.1
2003	8.23
2004	8.37
2005	8.51
2006	8.65
2007	8.79
2008	8.92
2009	9.06
2010	9.2
2011	9.34
2012	9.48
2013	9.62
2014	9.76
2015	9.9