Year	Years
1970	4.06
1971	4.2
1972	4.35
1973	4.5
1974	4.66
1975	4.82
1976	4.99
1977	5.17
1978	5.35
1979	5.54
1980	5.73
1981	5.93
1982	6.13
1983	6.34
1984	6.56
1985	6.78
1986	7.01
1987	7.24
1988	7.48
1989	7.71
1990	7.96
1991	8.2
1992	8.44
1993	8.69
1994	8.93
1995	9.17
1996	9.39
1997	9.62
1998	9.84
1999	10.05
2000	10.27
2001	10.48
2002	10.68
2003	10.89
2004	11.09
2005	11.29
2006	11.49
2007	11.68
2008	11.87
2009	12.06
2010	12.24
2011	12.42
2012	12.6
2013	12.78
2014	12.95
2015	13.12