Year	Percent
1960	20.20796
1961	19.89526
1962	19.4231
1963	19.32404
1964	18.72965
1965	18.51204
1966	18.75314
1967	18.47605
1968	20.69952
1969	21.48336
1970	21.35311
1971	21.43413
1972	20.20481
1973	21.98006
1974	26.26225
1975	24.87284
1976	27.42864
1977	29.05328
1978	27.42999
1979	26.80965
1980	26.06322
1981	25.44052
1982	25.10051
1983	25.22723
1984	26.96867
1985	27.31391
1986	24.32733
1987	23.85594
1988	21.48393
1989	21.93989
1990	22.60719
1991	21.89584
1992	22.21366
1993	24.06297
1994	25.27033
1995	25.15979
1996	25.73705
1997	25.29409
1998	23.73326
1999	23.60552
2000	24.84375
2001	24.73787
2002	23.77348
2003	23.53464
2004	23.44272
2005	24.68512
2006	26.70812
2007	24.86279
2008	26.80139
2009	26.12877
2010	28.2141
2011	30.51028
2012	29.73223
2013	29.66602
2014	28.24755
2015	27.38131
2016	28.25523
2017	30.52863