Year	Percent
1960	36.25708
1961	34.95089
1962	33.77229
1963	34.46614
1964	36.02151
1965	35.71887
1966	35.81115
1967	36.79378
1968	37.86116
1969	37.14581
1970	36.47657
1971	34.77264
1972	34.9513
1973	37.40764
1974	39.85729
1975	36.10204
1976	35.97051
1977	34.28551
1978	35.98291
1979	39.25305
1980	42.75431
1981	42.78176
1982	40.95567
1983	41.26701
1984	42.2602
1985	41.98047
1986	33.58715
1987	31.76216
1988	32.46186
1989	37.32105
1990	39.41949
1991	39.20302
1992	37.13659
1993	37.15817
1994	37.33727
1995	37.18317
1996	39.94294
1997	40.55781
1998	36.90766
1999	38.6043
2000	45.71407
2001	44.95429
2002	40.39499
2003	39.64855
2004	41.11486
2005	43.42535
2006	44.66481
2007	43.28276
2008	45.88649
2009	39.19183
2010	39.7252
2011	41.22861
2012	40.57434
2013	39.13876
2014	38.78287
2015	37.71758
2016	34.13664
2017	35.47242