Year	Percent
1960	39.63737
1961	38.0673
1962	38.95465
1963	33.74544
1964	30.29652
1965	34.25564
1966	36.57294
1967	39.64942
1968	39.69829
1969	39.73353
1970	40.37498
1971	41.54895
1972	41.75548
1973	40.81618
1974	41.24037
1975	39.97963
1976	39.60883
1977	39.77797
1978	39.49031
1979	39.63326
1980	43.24991
1981	43.38436
1982	43.95131
1983	43.40786
1984	43.42616
1985	44.60188
1986	45.15957
1987	45.40314
1988	45.11839
1989	46.65346
1990	46.8589
1991	47.45221
1992	48.83469
1993	49.39926
1994	49.41987
1995	49.48815
1996	50.45074
1997	50.78082
1998	52.26031
1999	51.72522
2000	51.63245
2001	52.81702
2002	53.35907
2003	53.56589
2004	52.77874
2005	53.56326
2006	54.24233
2007	54.42251
2008	55.21615
2009	55.08361
2010	53.63048
2011	53.65972
2012	54.03102
2013	54.01984
2014	54.34596
2015	54.04269
2016	53.74993
2017	52.84476