Year	Percent
1960	21.10972
1961	20.00954
1962	19.92028
1963	19.07303
1964	18.96981
1965	19.01173
1966	18.69734
1967	18.52424
1968	21.30409
1969	22.70327
1970	23.85234
1971	22.55134
1972	23.63558
1973	23.58673
1974	25.66184
1975	22.11718
1976	23.52955
1977	26.83137
1978	27.83964
1979	29.55662
1980	30.79983
1981	31.26477
1982	28.85527
1983	28.68173
1984	29.46465
1985	27.96946
1986	25.62799
1987	24.98561
1988	23.89582
1989	23.01319
1990	22.13383
1991	21.25411
1992	25.52326
1993	31.36633
1994	34.02851
1995	35.76241
1996	36.32765
1997	37.80094
1998	37.4674
1999	37.60233
2000	42.08688
2001	39.71282
2002	39.08584
2003	37.2662
2004	38.5892
2005	40.25562
2006	43.17089
2007	43.99681
2008	45.07798
2009	36.27098
2010	38.67771
2011	39.15954
2012	39.48136
2013	38.81419
2014	37.22223
2015	36.49406
2016	36.03278
2017	38.5877