Year	Years
1970	4
1971	4.1
1972	4.21
1973	4.33
1974	4.44
1975	4.56
1976	4.67
1977	4.79
1978	4.92
1979	5.04
1980	5.17
1981	5.3
1982	5.43
1983	5.56
1984	5.69
1985	5.83
1986	5.96
1987	6.1
1988	6.24
1989	6.38
1990	6.53
1991	6.67
1992	6.82
1993	6.97
1994	7.12
1995	7.27
1996	7.41
1997	7.56
1998	7.71
1999	7.86
2000	8.02
2001	8.17
2002	8.33
2003	8.48
2004	8.64
2005	8.8
2006	8.96
2007	9.13
2008	9.29
2009	9.45
2010	9.62
2011	9.78
2012	9.95
2013	10.11
2014	10.28
2015	10.44