| 年份 | 分类 | 数值 |
| 2000 | Total : United States short tons/labor hour | 6.82 |
| 2001 | Total : United States short tons/labor hour | 6.80 |
| 2002 | Total : United States short tons/labor hour | 6.95 |
| 2003 | Total : United States short tons/labor hour | 6.80 |
| 2004 | Total : United States short tons/labor hour | 6.36 |
| 2005 | Total : United States short tons/labor hour | 6.26 |
| 2006 | Total : United States short tons/labor hour | 6.27 |
| 2007 | Total : United States short tons/labor hour | 5.96 |
| 2008 | Total : United States short tons/labor hour | 5.61 |
| 2009 | Total : United States short tons/labor hour | 5.55 |
| 2010 | Total : United States short tons/labor hour | 5.19 |
| 2011 | Total : United States short tons/labor hour | 5.19 |
| 2012 | Total : United States short tons/labor hour | 5.54 |
| 2013 | Total : United States short tons/labor hour | 5.96 |
| 2014 | Total : United States short tons/labor hour | 6.28 |