Year	Years
1970	3.27
1971	3.35
1972	3.43
1973	3.52
1974	3.61
1975	3.7
1976	3.79
1977	3.88
1978	3.98
1979	4.07
1980	4.17
1981	4.27
1982	4.38
1983	4.48
1984	4.59
1985	4.7
1986	4.81
1987	4.93
1988	5.04
1989	5.16
1990	5.28
1991	5.41
1992	5.53
1993	5.66
1994	5.79
1995	5.92
1996	6.06
1997	6.2
1998	6.34
1999	6.48
2000	6.61
2001	6.75
2002	6.89
2003	7.03
2004	7.18
2005	7.32
2006	7.46
2007	7.6
2008	7.74
2009	7.88
2010	8.02
2011	8.16
2012	8.3
2013	8.44
2014	8.58
2015	8.72