Year	Percent
1960	18.81321
1961	16.34306
1962	24.18396
1963	25.97789
1964	28.03237
1965	20.26799
1966	25.33794
1967	25.00966
1968	26.64257
1969	26.11134
1970	24.5927
1971	24.59794
1972	25.48181
1973	25.67919
1974	36.23188
1975	30.01695
1976	28.19001
1977	24.17867
1978	24.17058
1979	26.99316
1980	28.93983
1981	25.0172
1982	19.24912
1983	18.30202
1984	32.84481
1985	37.10246
1986	33.46451
1987	37.42455
1988	41.74731
1989	44.5268
1990	43.69122
1991	43.0131
1992	43.66105
1993	41.61356
1994	40.94937
1995	38.67124
1996	37.75578
1997	39.01095
1998	41.60142
1999	41.32165
2000	43.80257
2001	38.61995
2002	37.99927
2003	42.29651
2004	40.20395
2005	33.32242
2006	35.54157
2007	35.18521
2008	37.48532
2009	29.58135
2010	33.32418
2011	34.80269
2012	33.8055
2013	31.5146
2014	30.70679
2015	29.42132
2016	28.91719
2017	28.08949