Peterson and Peterson tested 24 psychology students in a laboratory on their recall of three letter consonant trigrams. Rehearsal was prevented by using a distractor task when asked to count backwards in threes from a specific number. They found that participants were able to recall 90% of the trigrams after a 3-second delay, but only 2% after an 18-second delay. This suggests that short-term memory has a limited duration of about 20 seconds when rehearsal is prevented.
