Year	Years
1970	3.81
1971	3.92
1972	4.03
1973	4.14
1974	4.25
1975	4.36
1976	4.47
1977	4.59
1978	4.71
1979	4.83
1980	4.95
1981	5.08
1982	5.21
1983	5.34
1984	5.47
1985	5.6
1986	5.73
1987	5.87
1988	6
1989	6.14
1990	6.28
1991	6.42
1992	6.57
1993	6.71
1994	6.85
1995	7
1996	7.15
1997	7.3
1998	7.45
1999	7.6
2000	7.76
2001	7.91
2002	8.07
2003	8.23
2004	8.39
2005	8.55
2006	8.71
2007	8.88
2008	9.04
2009	9.2
2010	9.37
2011	9.53
2012	9.7
2013	9.86
2014	10.03
2015	10.19