Year	Years
1970	3.2
1971	3.29
1972	3.38
1973	3.48
1974	3.58
1975	3.68
1976	3.79
1977	3.89
1978	4
1979	4.11
1980	4.22
1981	4.34
1982	4.46
1983	4.57
1984	4.7
1985	4.82
1986	4.94
1987	5.07
1988	5.2
1989	5.33
1990	5.46
1991	5.6
1992	5.73
1993	5.87
1994	6
1995	6.14
1996	6.28
1997	6.42
1998	6.56
1999	6.71
2000	6.86
2001	7.01
2002	7.16
2003	7.31
2004	7.47
2005	7.62
2006	7.78
2007	7.94
2008	8.1
2009	8.27
2010	8.43
2011	8.6
2012	8.76
2013	8.93
2014	9.1
2015	9.27