| 1 |
USPS, first-class letter |
Sep. 15, 2007 |
Metered mail versus postage stamps on outgoing mail and stickers versus no stickers on outside envelope |
| 2 |
USPS, first-class letter |
Oct. 6, 2007 |
No experiments |
| 3 |
Priority mail |
Oct. 20, 2007 |
Business reply mail versus two types of stamped return envelopes |
| 4 |
Federal Express |
Nov. 3, 2007 |
Business reply mail versus two types of stamped return envelopes |
| 5 |
USPS, first-class letter |
Dec. 8, 2007 |
Business reply mail versus two types of stamped return envelopes |