Year	Years
1970	4
1971	4.1
1972	4.19
1973	4.28
1974	4.37
1975	4.47
1976	4.57
1977	4.66
1978	4.76
1979	4.86
1980	4.95
1981	5.05
1982	5.15
1983	5.25
1984	5.35
1985	5.44
1986	5.54
1987	5.64
1988	5.74
1989	5.83
1990	5.93
1991	6.03
1992	6.13
1993	6.22
1994	6.32
1995	6.41
1996	6.51
1997	6.61
1998	6.71
1999	6.81
2000	6.91
2001	7.02
2002	7.12
2003	7.23
2004	7.34
2005	7.45
2006	7.57
2007	7.68
2008	7.8
2009	7.92
2010	8.04
2011	8.16
2012	8.28
2013	8.41
2014	8.54
2015	8.67