Year	Percent
1960	7.46025
1961	7.86502
1962	9.16079
1963	9.75357
1964	8.98463
1965	8.56915
1966	9.43467
1967	15.44251
1968	17.42801
1969	19.08932
1970	23.44706
1971	22.7998
1972	20.37102
1973	27.85609
1974	27.65529
1975	30.4638
1976	29.24559
1977	23.08721
1978	22.89113
1979	19.38469
1980	18.91934
1981	20.26227
1982	18.38821
1983	15.54689
1984	16.18223
1985	16.60064
1986	14.76771
1987	16.90014
1988	17.29844
1989	13.81918
1990	13.77601
1991	17.46577
1992	12.23171
1993	12.93868
1994	11.31396
1995	8.88099
1996	8.67568
1997	8.40853
1998	7.85117
1999	7.34428
2000	7.07651
2001	7.31772
2002	8.30539
2003	8.50303
2004	8.18552
2005	7.6371
2006	7.48265
2007	6.76274
2008	6.55633
2009	6.74836
2010	6.23822
2011	6.23733
2012	6.04196
2013	5.91545
2014	5.99228
2015	5.67026
2016	4.65413
2017	4.72554