Year	Percent
1953	1.9
1954	2.1
1955	2.3
1956	2.3
1957	2
1958	1.9
1959	2
1960	1.7
1961	1.6
1962	1.5
1963	1.3
1964	1.3
1965	1.3
1966	1.4
1967	1.4
1968	1.1
1969	1.1
1970	1
1971	1.1
1972	1.3
1973	1.2
1974	1.3
1975	1.7
1976	1.7
1977	1.8
1978	2
1979	2
1980	2
1981	2.1
1982	2.3
1983	2.6
1984	2.8
1985	2.7
1986	2.8
1987	2.8
1988	2.6
1989	2.3
1990	2.2
1991	2.2
1992	2.2
1993	2.6
1994	3
1995	3.2
1996	3.3
1997	3.4
1998	4
1999	4.5
2000	4.5
2001	4.7
2002	5.1
2003	4.9
2004	4.4
2005	4.2
2006	3.9
2007	3.7
2008	3.8
2009	4.8
2010	4.6
2011	4.141
2012	4
2013	3.7
2014	3.4
2015	3.1
2016	2.8
2017	2.7