Year	Years
1970	4.15
1971	4.26
1972	4.37
1973	4.49
1974	4.6
1975	4.72
1976	4.84
1977	4.97
1978	5.09
1979	5.22
1980	5.34
1981	5.47
1982	5.6
1983	5.74
1984	5.87
1985	6
1986	6.14
1987	6.28
1988	6.41
1989	6.55
1990	6.69
1991	6.83
1992	6.97
1993	7.11
1994	7.25
1995	7.39
1996	7.52
1997	7.66
1998	7.79
1999	7.93
2000	8.07
2001	8.22
2002	8.36
2003	8.51
2004	8.66
2005	8.81
2006	8.96
2007	9.11
2008	9.26
2009	9.42
2010	9.58
2011	9.73
2012	9.89
2013	10.05
2014	10.21
2015	10.37