I have been surprised how often that two people seem to get the beep in a row, but since my experiences are truly anecdotal regarding this, I cannot draw any conclusions from this.

In response to tkey75, is the beep a function of time or is it a function of the number of times people go through the WTMD? If it is a function of the number of people who go through the WTMD, then it should not matter how long after one goes through the WTMD before the next person goes through. If it is a function of time where the last person to go through the WTMD (excluding crew members and perhaps children) prior to the beep is selected, then one decreases one's odds of being selected by going through right before someone else.

Relating to LoganTSO's response, although TSA employees might not be able to share the percentage of people who are randomly selected from precheck body scanners, are there any frequent fliers that can speculate?