Year	Years
1970	4.2
1971	4.3
1972	4.39
1973	4.49
1974	4.59
1975	4.69
1976	4.79
1977	4.9
1978	5
1979	5.11
1980	5.22
1981	5.33
1982	5.44
1983	5.55
1984	5.67
1985	5.78
1986	5.9
1987	6.02
1988	6.14
1989	6.26
1990	6.39
1991	6.51
1992	6.64
1993	6.77
1994	6.9
1995	7.03
1996	7.17
1997	7.31
1998	7.45
1999	7.59
2000	7.73
2001	7.87
2002	8.02
2003	8.16
2004	8.31
2005	8.45
2006	8.6
2007	8.74
2008	8.89
2009	9.03
2010	9.18
2011	9.33
2012	9.47
2013	9.62
2014	9.76
2015	9.91