Year	Years
1970	3.08
1971	3.17
1972	3.26
1973	3.36
1974	3.45
1975	3.55
1976	3.65
1977	3.75
1978	3.85
1979	3.96
1980	4.07
1981	4.18
1982	4.29
1983	4.41
1984	4.52
1985	4.64
1986	4.77
1987	4.89
1988	5.02
1989	5.15
1990	5.28
1991	5.41
1992	5.55
1993	5.69
1994	5.83
1995	5.98
1996	6.13
1997	6.28
1998	6.44
1999	6.6
2000	6.76
2001	6.92
2002	7.08
2003	7.25
2004	7.42
2005	7.58
2006	7.75
2007	7.92
2008	8.1
2009	8.27
2010	8.44
2011	8.61
2012	8.79
2013	8.96
2014	9.14
2015	9.31