Year	Percent
1960	9.06309
1961	8.72627
1962	8.47033
1963	8.2903
1964	8.1881
1965	8.14862
1966	8.10911
1967	8.01822
1968	7.91411
1969	7.75552
1970	7.59075
1971	7.40999
1972	5.07982
1973	4.70364
1974	4.55172
1975	4.38051
1976	4.2105
1977	4.05367
1978	3.8761
1979	3.70267
1980	3.53156
1981	3.34508
1982	3.20557
1983	3.10312
1984	3.05796
1985	3.06693
1986	3.06315
1987	3.0627
1988	3.06201
1989	3.06764
1990	3.07293
1991	3.07385
1992	3.5842
1993	3.58414
1994	3.49104
1995	3.37082
1996	3.24096
1997	3.11162
1998	2.99717
1999	2.89824
2000	2.80649
2001	2.72576
2002	2.51348
2003	2.38193
2004	2.23447
2005	2.06106
2006	1.88362
2007	1.73118
2008	1.64953
2009	1.66328
2010	1.73806
2011	1.83179
2012	1.90338
2013	1.93174
2014	1.90499
2015	1.84064
2016	1.77086
2017	1.70867