Duration - 1
Duration refers to how long a memory lasts before it is no longer available. STM does not last very long. To keep information in our STM is to rehearse it through verbal rehearsal and this information will eventually get transferred into LTM.
LTM refers to memories that last from 2 hours to 100 years plus. Shepard tested duration of LTM. He showed participants 612 memorable pictures one at a time. An hour later they were shown some of these pictures among a set of others and showed almost perfect recognition. Four months later they were still able to recognise 50% of the photographs.
Duration - 2
Peterson and Peterson conducted a landmark study of duration in STM. They enlisted the help of 24 students attending their university. Recall was tested as follows, the experimenter said a nonsense trigram followed by a 3 digit number (ADG 345). The trigram was set to have no meaning what so ever. Immediately after hearing the trigram and number the participant had to count back from this number in 3s or 4s until told to stop. Then the participant was asked to recall the trigram. The reason for counting backwards was to stop the participant from rehearsing the trigram as rehearsal aids recall. Each participant was given 2 practise trials and then 8 trials. On each trial the rention interval was different: 3,6,9,12,15 or 18. Participants remembered about 90% when there was only a 3 second retention interval and about 2% when there was an 18 second retention interval. This suggests that when rehearsal is prevented, STM lasts about 20 seconds at the most.