Peterson and Peterson: tested 24 undergraduate students.
The students were given trigrams (YGC, TFS, RQI) to
remember and also given a 3-digit number. They then had to
count this digit backwards, to prevent mental rehearsal. One
each trial, the participants stop for a different amount of
time (3, 6, 9, 12, 15 or 18 seconds etc). The participants
tended to forget the trigrams more easily after 18 seconds.