Year	Years
1970	3.98
1971	4.06
1972	4.15
1973	4.24
1974	4.33
1975	4.41
1976	4.5
1977	4.58
1978	4.67
1979	4.75
1980	4.83
1981	4.91
1982	4.98
1983	5.05
1984	5.12
1985	5.18
1986	5.24
1987	5.29
1988	5.34
1989	5.38
1990	5.41
1991	5.43
1992	5.45
1993	5.46
1994	5.46
1995	5.46
1996	5.54
1997	5.63
1998	5.73
1999	5.83
2000	5.93
2001	6.04
2002	6.16
2003	6.28
2004	6.4
2005	6.53
2006	6.66
2007	6.79
2008	6.92
2009	7.06
2010	7.2
2011	7.35
2012	7.49
2013	7.64
2014	7.79
2015	7.94