Year	Percent
1960	23.08216
1961	22.96479
1962	22.81383
1963	22.65719
1964	22.49615
1965	22.34006
1966	22.17356
1967	21.99819
1968	21.87404
1969	21.78848
1970	21.7024
1971	21.58045
1972	21.33497
1973	21.09834
1974	20.9444
1975	20.88234
1976	20.80185
1977	20.67747
1978	20.57822
1979	20.4978
1980	20.38272
1981	20.23412
1982	20.23081
1983	20.27473
1984	20.28725
1985	20.28894
1986	20.28726
1987	20.28565
1988	20.26817
1989	20.18822
1990	20.04605
1991	19.85818
1992	19.65391
1993	19.50546
1994	19.44345
1995	19.42662
1996	19.41334
1997	19.40423
1998	19.39585
1999	19.37102
2000	19.33738
2001	19.30892
2002	19.48111
2003	19.65578
2004	19.80599
2005	19.94445
2006	20.03722
2007	20.14251
2008	20.31635
2009	20.5015
2010	20.69312
2011	20.86635
2012	21.01639
2013	21.13859
2014	21.22102
2015	21.23159
2016	21.25092
2017	21.32377