Year	Percent
1960	22.69861
1961	24.77807
1962	21.09284
1963	20.07883
1964	19.13707
1965	16.89008
1966	19.37345
1967	20.17385
1968	20.05436
1969	18.83219
1970	21.57934
1971	19.72592
1972	19.22296
1973	22.10602
1974	28.19261
1975	33.34597
1976	36.72939
1977	37.3212
1978	30.06835
1979	29.86461
1980	28.00613
1981	34.76484
1982	33.73553
1983	29.8594
1984	34.7285
1985	33.80185
1986	28.39824
1987	27.17625
1988	25.14106
1989	27.79591
1990	30.06737
1991	27.46053
1992	28.09116
1993	27.60771
1994	26.37505
1995	29.04181
1996	25.25957
1997	27.22604
1998	27.39388
1999	28.83232
2000	32.3744
2001	31.20884
2002	31.61647
2003	30.82397
2004	33.48893
2005	36.9957
2006	38.74093
2007	43.91549
2008	49.93047
2009	39.91193
2010	43.01462
2011	48.72677
2012	50.19537
2013	47.24292
2014	46.84442
2015	42.08494
2016	45.33062
2017	46.6806