Year	Percent
1988-01-01	22.69
1989-01-01	24.11
1990-01-01	23.52
1991-01-01	29.07
1992-01-01	29.31
1993-01-01	34.35
1994-01-01	30.73
1995-01-01	32.56
1996-01-01	33.07
1997-01-01	35.09
1998-01-01	37.65
1999-01-01	38.11
2000-01-01	39.06
2001-01-01	38.15
2002-01-01	36.76
2003-01-01	36.60
2004-01-01	37.38
2005-01-01	36.17
2006-01-01	36.57
2007-01-01	35.48
2008-01-01	28.26
2009-01-01	24.93
2010-01-01	23.68
2011-01-01	22.42
2012-01-01	22.71
2013-01-01	23.47
2014-01-01	23.75
2015-01-01	23.90