Year	Percent
1960	24.56108
1961	21.29939
1962	20.65898
1963	23.1912
1964	25.51957
1965	26.77866
1966	23.13881
1967	24.22518
1968	22.22953
1969	22.46788
1970	24.63515
1971	25.3524
1972	22.17079
1973	22.10034
1974	27.70892
1975	29.23437
1976	28.06874
1977	24.45048
1978	24.87267
1979	25.00258
1980	26.54577
1981	29.41741
1982	26.02022
1983	20.34706
1984	22.92733
1985	21.86959
1986	21.07733
1987	19.70511
1988	21.90257
1989	20.72376
1990	18.18772
1991	16.95277
1992	16.78372
1993	17.29819
1994	19.29477
1995	21.47534
1996	22.58707
1997	22.85793
1998	23.89156
1999	22.16668
2000	24.27889
2001	25.42681
2002	27.9838
2003	24.51677
2004	25.61094
2005	26.70245
2006	31.00337
2007	32.50924
2008	37.24295
2009	27.50637
2010	27.37376
2011	29.65169
2012	31.17582
2013	33.27042
2014	32.96586
2015	31.4441
2016	30.08793
2017	28.40674