Year	Percent
1960	21.88135
1961	20.93064
1962	19.81878
1963	23.99346
1964	25.29015
1965	27.27273
1966	24.78187
1967	27.4464
1968	25.40652
1969	26.23895
1970	28.55731
1971	30.95647
1972	26.53186
1973	26.65519
1974	30.03199
1975	32.3423
1976	30.3811
1977	29.74624
1978	26.6688
1979	27.37798
1980	31.35638
1981	34.11518
1982	26.52872
1983	27.17322
1984	25.54336
1985	22.04958
1986	21.02094
1987	17.78428
1988	21.2283
1989	22.74589
1990	19.41325
1991	19.02497
1992	16.76861
1993	15.16232
1994	17.7201
1995	19.16665
1996	18.04003
1997	17.71512
1998	17.99154
1999	17.0351
2000	16.36533
2001	15.74458
2002	16.27797
2003	17.10588
2004	18.4666
2005	18.31498
2006	20.18299
2007	20.98552
2008	23.15017
2009	20.7049
2010	19.51297
2011	19.72094
2012	19.96597
2013	21.16356
2014	20.49948
2015	20.97968
2016	19.35308
2017	18.60187