Year	Years
1970	2.32
1971	2.39
1972	2.46
1973	2.54
1974	2.62
1975	2.7
1976	2.78
1977	2.87
1978	2.96
1979	3.05
1980	3.14
1981	3.23
1982	3.33
1983	3.43
1984	3.53
1985	3.63
1986	3.74
1987	3.85
1988	3.96
1989	4.07
1990	4.19
1991	4.3
1992	4.42
1993	4.54
1994	4.67
1995	4.8
1996	4.92
1997	5.06
1998	5.19
1999	5.33
2000	5.46
2001	5.6
2002	5.74
2003	5.89
2004	6.03
2005	6.18
2006	6.32
2007	6.47
2008	6.62
2009	6.77
2010	6.93
2011	7.08
2012	7.24
2013	7.39
2014	7.55
2015	7.71