Year	Percent
1960	31.26821
1961	31.64851
1962	31.07944
1963	31.57307
1964	31.33152
1965	31.10934
1966	30.943
1967	31.50429
1968	30.59829
1969	30.20083
1970	31.8863
1971	32.36022
1972	33.51957
1973	34.12063
1974	34.49356
1975	34.99954
1976	36.15096
1977	36.83577
1978	36.87914
1979	37.47117
1980	38.78785
1981	39.17279
1982	38.82816
1983	39.23328
1984	37.90776
1985	35.06801
1986	34.57587
1987	34.43269
1988	35.15557
1989	34.89828
1990	34.47217
1991	34.01444
1992	32.83712
1993	32.68322
1994	32.52989
1995	32.06258
1996	32.08591
1997	32.13303
1998	34.3902
1999	33.05588
2000	34.45607
2001	34.49931
2002	34.58374
2003	34.56093
2004	33.75262
2005	33.83416
2006	33.49273
2007	33.05304
2008	32.87777
2009	31.71018
2010	32.56828
2011	31.34718
2012	31.24629
2013	31.11578
2014	31.33398
2015	30.89633
2016	30.74671
2017	30.4523