Year	Years
1970	3.8
1971	3.9
1972	4.1
1973	4.2
1974	4.4
1975	4.5
1976	4.7
1977	4.8
1978	5
1979	5.2
1980	5.3
1981	5.5
1982	5.7
1983	5.8
1984	6
1985	6.2
1986	6.4
1987	6.5
1988	6.7
1989	6.9
1990	7
1991	7.2
1992	7.4
1993	7.5
1994	7.7
1995	7.8
1996	8
1997	8.1
1998	8.3
1999	8.5
2000	8.6
2001	8.8
2002	8.9
2003	9.1
2004	9.2
2005	9.3
2006	9.5
2007	9.6
2008	9.8
2009	9.9