Year	Years
1970	3.29
1971	3.37
1972	3.45
1973	3.52
1974	3.6
1975	3.67
1976	3.75
1977	3.82
1978	3.89
1979	3.96
1980	4.04
1981	4.11
1982	4.18
1983	4.25
1984	4.31
1985	4.38
1986	4.45
1987	4.52
1988	4.58
1989	4.65
1990	4.71
1991	4.77
1992	4.84
1993	4.9
1994	4.96
1995	5.02
1996	5.12
1997	5.22
1998	5.33
1999	5.43
2000	5.54
2001	5.66
2002	5.78
2003	5.9
2004	6.02
2005	6.14
2006	6.27
2007	6.4
2008	6.53
2009	6.67
2010	6.81
2011	6.95
2012	7.09
2013	7.23
2014	7.38
2015	7.53