Year	Years
1970	2.2
1971	2.2
1972	2.3
1973	2.4
1974	2.5
1975	2.6
1976	2.7
1977	2.7
1978	2.8
1979	2.9
1980	3
1981	3.1
1982	3.2
1983	3.4
1984	3.5
1985	3.6
1986	3.7
1987	3.9
1988	4
1989	4.2
1990	4.3
1991	4.5
1992	4.6
1993	4.8
1994	4.9
1995	5.1
1996	5.3
1997	5.4
1998	5.6
1999	5.8
2000	5.9
2001	6.1
2002	6.3
2003	6.4
2004	6.6
2005	6.8
2006	7
2007	7.2
2008	7.3
2009	7.5