Year	Percent
1960	38.26601
1961	38.47786
1962	38.65131
1963	36.88083
1964	36.64913
1965	41.15306
1966	40.71463
1967	42.17703
1968	43.33304
1969	43.15688
1970	44.54656
1971	40.78716
1972	40.91532
1973	39.79468
1974	41.70642
1975	40.39886
1976	38.74915
1977	43.90615
1978	42.77555
1979	47.02201
1980	48.46614
1981	48.44831
1982	50.48554
1983	49.86295
1984	49.42898
1985	43.2161
1986	45.03136
1987	45.94705
1988	45.31413
1989	45.85113
1990	47.5926
1991	46.35862
1992	46.40654
1993	46.2776
1994	39.39366
1995	39.53282
1996	37.47487
1997	38.33332
1998	37.98752
1999	39.3291
2000	43.79141
2001	41.55182
2002	41.95689
2003	39.7991
2004	41.64932
2005	40.20594
2006	43.06408
2007	45.81329
2008	41.53204
2009	42.35713
2010	38.84237
2011	36.62107
2012	37.44652
2013	40.05936
2014	40.33419
2015	41.99115
2016	42.05404
2017	42.01651