Year	Percent
1960	37.34866
1961	37.13225
1962	35.51213
1963	35.91044
1964	35.6679
1965	36.01879
1966	36.64972
1967	37.90756
1968	35.61533
1969	34.76738
1970	37.09931
1971	37.31304
1972	34.19492
1973	37.7917
1974	42.53918
1975	41.88503
1976	44.32819
1977	43.38637
1978	35.62127
1979	36.92352
1980	36.72972
1981	35.57846
1982	35.89835
1983	33.48152
1984	33.51603
1985	34.76072
1986	36.97185
1987	33.97141
1988	33.04582
1989	33.84295
1990	33.17006
1991	31.47299
1992	30.53107
1993	30.89081
1994	31.39116
1995	31.14334
1996	31.17857
1997	32.50576
1998	35.11424
1999	31.40363
2000	28.90769
2001	28.31211
2002	27.25203
2003	26.93514
2004	27.93414
2005	27.42544
2006	27.72386
2007	29.87491
2008	28.95358
2009	27.78803
2010	28.46489
2011	28.36822
2012	27.54178
2013	28.35444
2014	29.78328
2015	32.09207
2016	33.27319
2017	33.05191