Year	Years
1970	3.6
1971	3.7
1972	3.8
1973	3.9
1974	4.1
1975	4.2
1976	4.4
1977	4.5
1978	4.6
1979	4.8
1980	4.9
1981	5.1
1982	5.3
1983	5.4
1984	5.6
1985	5.7
1986	5.9
1987	6.1
1988	6.2
1989	6.4
1990	6.6
1991	6.8
1992	7
1993	7.1
1994	7.3
1995	7.5
1996	7.7
1997	7.9
1998	8.1
1999	8.2
2000	8.4
2001	8.6
2002	8.8
2003	9
2004	9.1
2005	9.3
2006	9.5
2007	9.6
2008	9.8
2009	10