Year	Percent
1960	33.81995
1961	33.82176
1962	35.75085
1963	37.69944
1964	38.43191
1965	38.04492
1966	37.35594
1967	37.92011
1968	39.37282
1969	40.72707
1970	39.4687
1971	36.91122
1972	35.94712
1973	38.98832
1974	46.65115
1975	36.11432
1976	38.10941
1977	34.55173
1978	34.66878
1979	35.45956
1980	35.42351
1981	34.65585
1982	33.4622
1983	36.92985
1984	37.77774
1985	33.87589
1986	33.17438
1987	33.66435
1988	38.21192
1989	36.80234
1990	35.91044
1991	34.22703
1992	32.13132
1993	31.57117
1994	32.67794
1995	33.78005
1996	34.27898
1997	33.79268
1998	30.90612
1999	30.5509
2000	31.4223
2001	31.82258
2002	31.966
2003	33.41272
2004	36.00057
2005	36.85919
2006	41.10699
2007	39.94378
2008	34.23929
2009	34.44018
2010	35.69509
2011	34.70132
2012	32.53912
2013	31.19376
2014	30.84803
2015	29.67374
2016	28.92954
2017	29.95528