Year	Percent
1953	1.9
1954	2.3
1955	2.5
1956	2.3
1957	1.9
1958	2.1
1959	2.2
1960	1.7
1961	1.4
1962	1.3
1963	1.3
1964	1.1
1965	1.2
1966	1.3
1967	1.3
1968	1.2
1969	1.1
1970	1.1
1971	1.2
1972	1.4
1973	1.3
1974	1.4
1975	1.9
1976	2
1977	2
1978	2.2
1979	2.1
1980	2
1981	2.2
1982	2.4
1983	2.6
1984	2.7
1985	2.6
1986	2.8
1987	2.8
1988	2.5
1989	2.3
1990	2.1
1991	2.1
1992	2.2
1993	2.5
1994	2.9
1995	3.2
1996	3.4
1997	3.4
1998	4.1
1999	4.7
2000	4.75
2001	5.02
2002	5.39
2003	5.25
2004	4.73
2005	4.45
2006	4.19
2007	3.89
2008	4
2009	5.07
2010	5.1
2011	4.55
2012	4.36
2013	4
2014	3.6
2015	3.4
2016	3.1
2017	2.8
2018	2.4
2019	2.4
2020	2.8