Year	Percent
1960	16.15923
1961	15.86447
1962	16.3345
1963	17.38259
1964	18.38397
1965	19.63488
1966	18.14492
1967	17.45679
1968	17.78469
1969	18.08306
1970	15.22702
1971	15.16234
1972	16.18775
1973	20.09285
1974	21.2612
1975	21.18133
1976	20.83001
1977	20.68136
1978	19.48386
1979	20.35843
1980	25.26175
1981	25.75087
1982	24.64243
1983	24.6994
1984	24.09752
1985	24.25418
1986	24.95752
1987	24.19428
1988	23.29866
1989	24.8381
1990	25.34972
1991	24.39315
1992	23.94375
1993	22.97813
1994	21.71751
1995	22.6826
1996	23.20928
1997	23.05196
1998	25.9717
1999	28.12152
2000	34.6973
2001	33.35122
2002	30.23614
2003	29.64547
2004	29.19033
2005	29.58863
2006	31.67623
2007	35.0041
2008	35.96815
2009	28.76166
2010	30.72689
2011	32.3092
2012	33.13377
2013	33.16571
2014	34.77641
2015	31.67098
2016	31.18114
2017	34.29633