Year	Percent
1960	20.102
1961	20.11901
1962	20.11289
1963	20.09662
1964	20.08857
1965	20.09889
1966	20.13404
1967	20.19011
1968	20.26191
1969	20.339
1970	20.41843
1971	20.25649
1972	20.04762
1973	19.85633
1974	19.69651
1975	19.57599
1976	19.49485
1977	19.44713
1978	19.4288
1979	19.43316
1980	19.45442
1981	19.49346
1982	19.59676
1983	19.71357
1984	19.81343
1985	19.88293
1986	19.91619
1987	19.91909
1988	19.90788
1989	19.90409
1990	19.92407
1991	19.90436
1992	19.83983
1993	19.79267
1994	19.7542
1995	19.71813
1996	19.6836
1997	19.65302
1998	19.6281
1999	19.61103
2000	19.60349
2001	19.60466
2002	19.61374
2003	19.5239
2004	19.41849
2005	19.32203
2006	19.23508
2007	19.15622
2008	19.07993
2009	18.99946
2010	18.90989
2011	18.80894
2012	18.69881
2013	18.5855
2014	18.52708
2015	18.52907
2016	18.54267
2017	18.56573