Year	Percent
1960	4.98714
1961	6.2119
1962	7.78011
1963	7.82183
1964	7.90124
1965	7.97538
1966	8.07473
1967	8.13561
1968	8.10961
1969	7.9396
1970	7.72299
1971	8.25204
1972	7.99024
1973	7.75016
1974	7.60682
1975	7.50069
1976	7.40745
1977	7.26791
1978	7.1437
1979	6.98756
1980	6.83018
1981	6.64758
1982	4.9232
1983	4.89445
1984	4.87048
1985	4.83419
1986	4.80223
1987	4.75898
1988	4.91526
1989	4.80998
1990	4.6974
1991	4.58459
1992	4.47838
1993	4.36252
1994	3.85392
1995	3.74019
1996	3.67168
1997	3.61032
1998	3.55952
1999	3.5161
2000	3.48598
2001	3.44906
2002	3.43388
2003	3.4587
2004	3.53228
2005	3.6338
2006	3.72122
2007	3.78443
2008	3.85557
2009	3.92815
2010	3.99144
2011	4.06112
2012	4.08582
2013	3.98396
2014	3.7316
2015	3.3936
2016	3.03702
2017	2.73461