Year	Years
1970	3.8
1971	3.9
1972	4.1
1973	4.2
1974	4.3
1975	4.4
1976	4.6
1977	4.7
1978	4.8
1979	5
1980	5.1
1981	5.3
1982	5.4
1983	5.6
1984	5.7
1985	5.9
1986	6
1987	6.2
1988	6.3
1989	6.5
1990	6.6
1991	6.8
1992	7
1993	7.1
1994	7.3
1995	7.4
1996	7.6
1997	7.7
1998	7.9
1999	8.1
2000	8.2
2001	8.4
2002	8.5
2003	8.7
2004	8.8
2005	9
2006	9.1
2007	9.3
2008	9.4
2009	9.6