Year	Percent
1960	18.62069
1961	14.77273
1962	14.43299
1963	12.01717
1964	11.34328
1965	11.47541
1966	11.48718
1967	13.17829
1968	12.20568
1969	12.13036
1970	15.20755
1971	11.4335
1972	14.89417
1973	9.50559
1974	16.89075
1975	20.27809
1976	20.07877
1977	22.65854
1978	21.15458
1979	21.16702
1980	20.63143
1981	19.03135
1982	17.29079
1983	23.60387
1984	21.37545
1985	21.10004
1986	20.26865
1987	19.20595
1988	17.73593
1989	17.69969
1990	18.09635
1991	17.86124
1992	19.62547
1993	19.55895
1994	20.38202
1995	19.10086
1996	19.85846
1997	20.24855
1998	19.2201
1999	18.2888
2000	20.01564
2001	19.50485
2002	19.40739
2003	24.32678
2004	29.36439
2005	28.47368
2006	31.67407
2007	30.11996
2008	35.00507
2009	26.2952
2010	25.35604
2011	26.82488
2012	29.14152
2013	26.36471
2014	25.54496
2015	22.85039
2016	19.88268
2017	18.42955