Year	Percent
1960	7.43969
1961	7.47431
1962	7.53372
1963	7.58925
1964	7.64297
1965	7.68439
1966	7.73779
1967	7.77108
1968	7.79581
1969	7.78654
1970	7.77871
1971	7.75115
1972	7.74297
1973	7.70946
1974	7.70968
1975	7.702
1976	7.70592
1977	7.67111
1978	7.65449
1979	6.95359
1980	5.14447
1981	5.15699
1982	5.18305
1983	5.20065
1984	5.21277
1985	5.2177
1986	5.21677
1987	5.21221
1988	5.25452
1989	5.33423
1990	5.44874
1991	5.56278
1992	5.11113
1993	4.35954
1994	4.28304
1995	4.14328
1996	3.99325
1997	3.87183
1998	3.795
1999	3.78649
2000	3.81702
2001	3.84733
2002	4.03685
2003	4.31242
2004	4.26956
2005	4.20329
2006	4.13789
2007	4.08642
2008	4.04564
2009	4.01668
2010	4.0037
2011	3.99143
2012	3.97562
2013	3.95511
2014	3.94923
2015	3.93741
2016	3.92539
2017	3.91149