Year	Percent
1965	27.78787
1966	27.01409
1967	28.03652
1968	28.22992
1969	28.36083
1970	29.9876
1971	29.41201
1972	30.13397
1973	32.30264
1974	32.26248
1975	31.85949
1976	31.62712
1977	30.04354
1978	29.88587
1979	31.67948
1980	27.85107
1981	25.21846
1982	22.69867
1983	25.64145
1984	29.48935
1985	25.64079
1986	28.46941
1987	29.55986
1988	30.06165
1989	31.49238
1990	29.80367
1991	33.20055
1992	37.21214
1993	30.46303
1994	29.15294
1995	25.70024
1996	23.00239
1997	22.53365
1998	20.80615
1999	19.11642
2004	24.38275
2005	26.47798
2006	30.65078
2007	32.46206
2008	30.48296
2009	23.69853
2010	24.17514
2011	24.99894
2012	23.22504
2013	22.87741
2014	22.52228
2015	21.15958
2016	20.19118
2017	21.54515