Year	Percent
1960	20.68716
1961	20.55453
1962	21.08622
1963	20.69277
1964	20.39137
1965	21.55253
1966	20.31833
1967	19.99372
1968	21.45723
1969	23.27557
1970	24.18126
1971	25.07242
1972	24.55988
1973	25.72915
1974	25.34178
1975	26.68756
1976	27.37744
1977	28.94349
1978	27.5349
1979	28.5258
1980	29.92491
1981	28.25296
1982	26.5174
1983	26.53984
1984	26.49695
1985	26.54764
1986	26.95548
1987	27.78084
1988	27.0321
1989	27.10101
1990	26.32761
1991	25.89559
1992	25.88954
1993	25.84955
1994	26.40419
1995	26.68398
1996	26.57423
1997	26.99366
1998	27.61671
1999	27.36132
2000	27.31808
2001	26.82059
2002	28.01061
2003	28.42457
2004	28.6181
2005	30.18809
2006	30.6423
2007	29.91982
2008	29.37119
2009	29.67144
2010	26.64416
2011	27.99567
2012	30.13006
2013	29.16276
2014	28.298
2015	27.16953
2016	27.28693
2017	27.20136