Year	Years
1970	3.47
1971	3.57
1972	3.68
1973	3.78
1974	3.89
1975	4
1976	4.12
1977	4.24
1978	4.35
1979	4.48
1980	4.6
1981	4.73
1982	4.86
1983	4.99
1984	5.12
1985	5.26
1986	5.4
1987	5.54
1988	5.69
1989	5.84
1990	5.99
1991	6.14
1992	6.29
1993	6.45
1994	6.61
1995	6.77
1996	6.93
1997	7.1
1998	7.26
1999	7.43
2000	7.59
2001	7.76
2002	7.93
2003	8.1
2004	8.26
2005	8.43
2006	8.6
2007	8.77
2008	8.94
2009	9.11
2010	9.28
2011	9.45
2012	9.62
2013	9.79
2014	9.96
2015	10.13