Year	Percent
1960-01-01	4.870
1961-01-01	4.745
1962-01-01	4.480
1963-01-01	5.020
1964-01-01	3.980
1965-01-01	3.570
1966-01-01	3.325
1967-01-01	3.630
1968-01-01	3.690
1969-01-01	3.595
1970-01-01	3.580
1971-01-01	3.610
1972-01-01	3.610
1973-01-01	3.780
1974-01-01	3.350
1975-01-01	4.015
1976-01-01	4.430
1977-01-01	4.610
1978-01-01	4.950
1979-01-01	4.880
1980-01-01	4.780
1981-01-01	5.470
1982-01-01	6.810
1983-01-01	8.430
1984-01-01	7.535
1985-01-01	7.620
1986-01-01	7.590
1987-01-01	6.430
1988-01-01	5.860
1989-01-01	5.750
1990-01-01	5.400
1991-01-01	5.720
1992-01-01	5.380
1993-01-01	4.970
1994-01-01	4.715
1995-01-01	4.355
1996-01-01	3.995
1997-01-01	3.575
1998-01-01	4.420
1999-01-01	4.230
2000-01-01	3.745
2001-01-01	3.645
2002-01-01	3.450
2003-01-01	3.420
2004-01-01	2.715
2005-01-01	2.625
2006-01-01	2.440
2007-01-01	2.130
2008-01-01	1.930
2009-01-01	2.460
2010-01-01	2.340
2011-01-01	2.480
2012-01-01	2.300
2013-01-01	2.030
2014-01-01	1.870
2015-01-01	2.210