Year	Years
1970	4.5
1971	4.6
1972	4.8
1973	4.9
1974	5
1975	5.2
1976	5.3
1977	5.4
1978	5.6
1979	5.7
1980	5.8
1981	5.9
1982	6.1
1983	6.2
1984	6.3
1985	6.5
1986	6.6
1987	6.7
1988	6.8
1989	6.9
1990	7.1
1991	7.2
1992	7.3
1993	7.4
1994	7.6
1995	7.7
1996	7.8
1997	7.9
1998	8
1999	8.2
2000	8.3
2001	8.4
2002	8.5
2003	8.6
2004	8.8
2005	8.9
2006	9
2007	9.1
2008	9.2
2009	9.4