Year	Percent
1960	6.59535
1961	6.7177
1962	6.83171
1963	7.82231
1964	8.67609
1965	8.5091
1966	8.34647
1967	8.22947
1968	8.19769
1969	8.24574
1970	8.36744
1971	8.49865
1972	8.59509
1973	8.62004
1974	8.59033
1975	7.9598
1976	7.73402
1977	7.65989
1978	7.7214
1979	7.92306
1980	8.16568
1981	8.36389
1982	8.41399
1983	8.2384
1984	7.8266
1985	7.27412
1986	6.73659
1987	5.554
1988	5.14772
1989	4.76425
1990	4.41545
1991	4.09216
1992	3.79441
1993	3.15905
1994	2.80605
1995	2.59995
1996	2.3948
1997	2.2359
1998	2.21149
1999	2.34382
2000	2.58002
2001	2.85038
2002	3.07297
2003	3.20556
2004	3.21545
2005	3.15006
2006	3.05236
2007	2.98063
2008	2.96106
2009	3.00709
2010	3.09065
2011	3.18419
2012	3.22452
2013	3.17381
2014	3.00692
2015	2.76897
2016	2.51676
2017	2.29656