Year	Percent
1960	26.17726
1961	25.39857
1962	26.34471
1963	27.43377
1964	28.75396
1965	33.48351
1966	30.84367
1967	31.62779
1968	32.91006
1969	32.33427
1970	34.98966
1971	37.64047
1972	37.10533
1973	37.22152
1974	48.34099
1975	38.54778
1976	34.91072
1977	36.33477
1978	36.03145
1979	37.19336
1980	36.81902
1981	48.17618
1982	42.16487
1983	30.85685
1984	29.03058
1985	28.72035
1986	26.95435
1987	30.73299
1988	31.5733
1989	34.09079
1990	36.28173
1991	38.17828
1992	40.98414
1993	43.52207
1994	42.33129
1995	41.88071
1996	44.27316
1997	46.23331
1998	48.31119
1999	45.16847
2000	43.78157
2001	40.50343
2002	41.37415
2003	42.8439
2004	43.2385
2005	45.97005
2006	46.73262
2007	46.11585
2008	47.88861
2009	35.33775
2010	35.04562
2011	36.59449
2012	35.97497
2013	34.28536
2014	34.79054
2015	31.69337
2016	31.96419
2017	33.42061