Year	Percent
1960	35.55187
1961	35.21765
1962	35.01756
1963	35.04874
1964	36.22014
1965	36.45828
1966	35.73754
1967	36.04469
1968	37.18191
1969	35.40078
1970	36.70942
1971	41.28485
1972	34.32599
1973	31.25081
1974	29.99072
1975	26.43481
1976	33.63634
1977	34.37287
1978	31.43998
1979	31.86153
1980	44.55567
1981	45.69144
1982	46.14478
1983	45.78258
1984	41.64745
1985	44.41744
1986	45.08442
1987	45.49177
1988	46.38867
1989	47.18134
1990	46.70069
1991	44.35734
1992	44.3838
1993	46.04466
1994	46.28378
1995	45.1285
1996	50.1242
1997	49.93418
1998	49.83363
1999	50.20074
2000	50.56632
2001	50.58012
2002	51.57242
2003	52.40049
2004	52.80586
2005	52.87996
2006	52.7446
2007	52.88402
2008	52.93212
2009	53.32332
2010	53.4983
2011	53.04579
2012	53.15158
2013	53.39018
2014	53.63575
2015	53.71384
2016	53.6713
2017	53.47621