At its most basic level, a police lineup involves placing a suspect among people not suspected of committing the crime (fillers) and asking the eyewitness if he or she can identify the perpetrator. This can be done using a live lineup of people or, as more commonly done in U.S. police departments, a lineup of photographs. Live lineups typically use five or six people (a suspect plus four or five fillers) and photo lineups six or more photographs.
I thought that in all police lineups, the eyewitness simultaneously evaluated a number of potential suspects. However, this is not the case–there are also sequential lineups:
During sequential lineups, on the other hand, witnesses must make a decision about each photograph or member before moving on to the next, prompting them to use “absolute judgment.” In other words, witnesses compare each photograph or person only to their memory of what the offender looked like.
Surprisingly, the double-blind sequential lineup is more reliable in a laboratory setting. However, a field test in Illinois had less reliable results — it had a higher rate of false identification and lower rate of true identification. There is some controversy about the Illinois field test, so this is not the final word.
The sequential lineups remind me of the secretary problem. There is one difference: since the real suspect may not be in the lineup, there may not be a “best” secretary. I wonder if there is a potential application area here.