Year	Percent
1960	31.55564
1961	33.30696
1962	32.75364
1963	30.49772
1964	28.60874
1965	27.03731
1966	26.05769
1967	25.45152
1968	26.35207
1969	23.02192
1970	22.48022
1971	19.97619
1972	18.94467
1973	19.37671
1974	21.89164
1975	22.75779
1976	21.2998
1977	24.87321
1978	24.93194
1979	33.27182
1980	35.68429
1981	29.63167
1982	28.44427
1983	30.26123
1984	28.56873
1985	28.67877
1986	32.28762
1987	27.35928
1988	37.63098
1989	24.43159
1990	22.20339
1991	17.71452
1992	20.60919
1993	22.02906
1994	22.68166
1995	24.68263
1996	25.05592
1997	26.53423
1998	25.18312
1999	25.77543
2000	27.91201
2001	27.5398
2002	27.76119
2003	29.80491
2004	34.31249
2005	39.2707
2006	44.15642
2007	47.43229
2008	50.57739
2009	40.36917
2010	44.62416
2011	48.9766
2012	46.69404
2013	43.0035
2014	40.71932
2015	38.08808
2016	38.24093
2017	40.10308