Year	Years
1970	4.6
1971	4.7
1972	4.9
1973	5
1974	5.2
1975	5.3
1976	5.5
1977	5.6
1978	5.8
1979	6
1980	6.1
1981	6.3
1982	6.5
1983	6.7
1984	6.8
1985	7
1986	7.2
1987	7.4
1988	7.5
1989	7.7
1990	7.9
1991	8
1992	8.2
1993	8.4
1994	8.6
1995	8.7
1996	8.9
1997	9
1998	9.2
1999	9.4
2000	9.5
2001	9.7
2002	9.8
2003	10
2004	10.1
2005	10.3
2006	10.4
2007	10.6
2008	10.7
2009	10.8