Upjohn Institute
| Year | Percent |
|---|---|
| 1990 | 0.46% |
| 1991 | 0.47% |
| 1992 | 0.52% |
| 1993 | 0.56% |
| 1994 | 0.64% |
| 1995 | 0.75% |
| 1996 | 0.82% |
| 1997 | 0.85% |
| 1998 | 0.89% |
| 1999 | 0.90% |
| 2000 | 1.01% |
| 2001 | 1.39% |
| 2002 | 1.38% |
| 2003 | 1.43% |
| 2004 | 1.50% |
| 2005 | 1.48% |
| 2006 | 1.48% |
| 2007 | 1.44% |
| 2008 | 1.42% |
| 2009 | 1.45% |
| 2010 | 1.37% |
| 2011 | 1.41% |
| 2012 | 1.40% |
| 2013 | 1.40% |
| 2014 | 1.43% |
| 2015 | 1.42% |