Year	Percent
1960	19.10718
1961	20.47156
1962	22.81541
1963	21.06494
1964	23.51214
1965	26.05096
1966	21.0804
1967	16.86518
1968	23.25179
1969	26.58869
1970	29.12902
1971	31.86764
1972	31.94152
1973	32.79528
1974	31.04367
1975	31.08236
1976	26.09068
1977	23.90805
1978	26.18407
1979	27.01021
1980	23.55232
1981	22.19785
1982	24.18851
1983	24.62348
1984	23.01055
1985	19.52617
1986	19.16769
1987	19.52826
1988	18.54843
1989	17.91243
1990	20.04024
1991	25.9532
1992	25.71975
1993	26.35888
1994	25.73745
1995	25.85209
1996	25.87669
1997	24.88884
1998	23.14394
1999	23.57729
2000	22.79906
2001	22.47663
2002	20.7652
2003	19.50564
2004	19.75872
2005	20.54592
2006	20.45782
2007	21.18156
2008	19.97508
2009	18.26405
2010	18.44132
2011	20.3992
2012	21.23676
2013	19.95883
2014	20.20738
2015	19.75567
2016	20.48309