Year	Percent
1968-01-01	23.6980008246884
1969-01-01	24.0180134009229
1970-01-01	24.2051939023371
1971-01-01	24.7460832793019
1972-01-01	24.7929379420525
1973-01-01	24.2289595429139
1974-01-01	24.8554753617066
1975-01-01	25.2017171738936
1976-01-01	24.8429477448113
1977-01-01	24.1575467186562
1978-01-01	23.5045810843280
1979-01-01	23.0123283437447
1980-01-01	22.8102075560917
1981-01-01	22.5721861922398
1982-01-01	22.0337889914715
1983-01-01	21.2450766743208
1984-01-01	20.9948804501486
1985-01-01	20.8993730438742
1986-01-01	20.6776059436913
1987-01-01	20.3754674295617
1988-01-01	19.8962127938150
1989-01-01	19.2973272738731
1990-01-01	18.8227622255584
1991-01-01	18.3474768613864
1992-01-01	18.1833092873939
1993-01-01	18.3709167803752
1994-01-01	18.4954481893481
1995-01-01	18.5111628232726
1996-01-01	18.1501266582013
1997-01-01	17.6714646667734
1998-01-01	17.8489223061115
1999-01-01	17.9517336921641
2000-01-01	17.9561704533949
2001-01-01	17.7505611864813
2002-01-01	17.8782745331828
2003-01-01	17.7118987175112
2004-01-01	17.6367997784437
2005-01-01	17.3916600412714
2006-01-01	17.0456770747311
2007-01-01	16.6162113346933
2008-01-01	16.4775824006535
2009-01-01	16.1184925603273
2010-01-01	15.8556690662915
2011-01-01	15.6886929622165
2012-01-01	15.9196572905508
2013-01-01	15.1410248060334
2014-01-01	14.7317190890518
2015-01-01	14.4246196735186
2016-01-01	13.8659908786137
2017-01-01	13.3416621560967