Year	Percent
1960	12.72851
1961	12.82887
1962	11.64744
1963	11.76046
1964	11.6404
1965	24.36489
1966	22.38244
1967	22.90411
1968	18.35771
1969	22.48403
1970	19.55992
1971	22.39885
1972	23.25352
1973	20.52541
1974	24.6432
1975	20.57484
1976	21.05576
1977	24.48968
1978	26.60325
1979	22.78524
1980	25.37488
1981	25.82636
1982	21.76482
1983	20.66604
1984	18.78183
1985	18.95161
1986	23.56885
1987	25.40759
1988	26.11011
1989	27.08992
1990	24.072
1991	22.04545
1992	22.67006
1993	21.07743
1994	21.28627
1995	19.82476
1996	18.53782
1997	20.30894
1998	23.99566
1999	19.62846
2000	21.27872
2001	22.34919
2002	23.70238
2003	19.58955
2004	20.19901
2005	21.6371
2006	22.45981
2007	22.70483
2008	26.38751
2009	25.63927
2010	28.03731
2011	28.1424
2012	27.79584
2013	28.46706
2014	28.31444
2015	26.8701
2016	25.15635
2017	25.36131