Year	Percent
1960	14.39628
1961	13.91919
1962	12.83299
1963	12.59577
1964	12.63685
1965	13.22922
1966	13.28841
1967	13.14493
1968	13.17425
1969	14.01162
1970	15.98116
1971	16.48168
1972	16.62311
1973	17.52472
1974	20.63028
1975	18.8332
1976	19.33517
1977	20.23357
1978	20.24267
1979	20.71595
1980	20.98258
1981	21.96986
1982	21.3987
1983	22.43633
1984	23.83548
1985	23.32421
1986	20.38348
1987	19.59735
1988	20.36673
1989	21.65815
1990	20.97573
1991	21.33488
1992	21.33056
1993	20.79258
1994	21.66032
1995	22.62293
1996	23.05556
1997	25.5232
1998	26.12649
1999	26.07654
2000	28.59497
2001	28.2659
2002	27.53105
2003	26.11197
2004	26.46884
2005	27.03305
2006	27.93506
2007	27.85326
2008	28.11938
2009	24.83555
2010	26.7883
2011	28.42134
2012	29.20303
2013	29.36474
2014	29.66668
2015	30.59262
2016	30.15546
2017	30.88185