Year	Percent
1960	16.32275
1961	16.82694
1962	17.55795
1963	18.74421
1964	21.21423
1965	22.11455
1966	22.19804
1967	22.91711
1968	23.82688
1969	24.63032
1970	26.89074
1971	28.44458
1972	30.8776
1973	30.51874
1974	31.15908
1975	31.20629
1976	31.97595
1977	31.75113
1978	31.4076
1979	33.08904
1980	34.8825
1981	34.43171
1982	33.33785
1983	34.43877
1984	35.16714
1985	32.18286
1986	33.13874
1987	32.62788
1988	33.59733
1989	32.13327
1990	30.84163
1991	32.5982
1992	32.26518
1993	32.11997
1994	31.29236
1995	31.47992
1996	31.53779
1997	31.04802
1998	32.04908
1999	30.82689
2000	32.46474
2001	30.35035
2002	30.62579
2003	30.04513
2004	31.60292
2005	30.91246
2006	30.38065
2007	27.66966
2008	25.89723
2009	26.4795
2010	26.09245
2011	24.73703
2012	24.63939
2013	23.15291
2014	24.0227
2015	24.24879
2016	23.72419
2017	23.21253