Year	Percent
1960	17.50043
1961	18.87823
1962	19.45334
1963	18.79443
1964	15.99412
1965	19.50363
1966	19.05839
1967	19.35734
1968	18.02719
1969	20.029
1970	20.53644
1971	24.33656
1972	22.02707
1973	23.99335
1974	26.73865
1975	21.6411
1976	23.66219
1977	24.26977
1978	23.46136
1979	25.31401
1980	26.57473
1981	29.02665
1982	24.68609
1983	15.88235
1984	14.70076
1985	13.75141
1986	16.071
1987	19.53554
1988	17.92206
1989	17.74298
1990	18.47805
1991	18.6538
1992	20.66244
1993	21.26666
1994	20.99541
1995	19.74293
1996	17.91179
1997	20.26459
1998	22.80616
1999	21.091
2000	20.73622
2001	20.14245
2002	19.9275
2003	19.00526
2004	18.9911
2005	19.05293
2006	21.21883
2007	23.46513
2008	24.86345
2009	18.43007
2010	19.71486
2011	19.88936
2012	20.4796
2013	18.97722
2014	18.80662
2015	18.43278
2016	17.9218
2017	17.99904