Year	Percent
1960	12.49748
1961	12.7924
1962	12.67058
1963	12.16489
1964	12.81826
1965	14.28918
1966	14.69829
1967	14.46318
1968	15.2408
1969	15.84412
1970	15.21085
1971	15.62867
1972	16.36443
1973	16.1557
1974	18.58182
1975	18.7907
1976	20.10725
1977	21.44638
1978	21.7464
1979	22.45508
1980	20.24908
1981	21.73295
1982	21.18587
1983	20.39636
1984	21.07015
1985	21.17847
1986	18.76228
1987	18.04016
1988	17.63284
1989	18.57443
1990	18.31641
1991	17.08917
1992	17.53373
1993	20.46275
1994	21.93933
1995	24.74947
1996	23.76293
1997	24.20028
1998	24.12269
1999	23.25049
2000	25.65779
2001	25.73175
2002	24.47218
2003	23.36074
2004	24.0629
2005	24.65166
2006	26.22797
2007	27.4272
2008	26.96454
2009	22.4766
2010	25.18821
2011	27.00635
2012	28.58636
2013	28.86259
2014	29.30651
2015	29.92601
2016	29.80153
2017	31.29653