Peterson and Peterson (1959)
Aim: To investigate the duration of STM.
Method: Participants shown nonsense trigrams (3 random consonants) and asked to recall them after 3,6,9,12,15 or 18 seconds. They were asked to count backwards in threes from a given number. This was an 'interference task' which prevented them from repeating the letters to themselves.
Results: After 3 seconds, participants could recall about 80% of trigrams correctly. After 18 seconds, only about 10% were recalled correctly.
Conclusion: Very little can stay in STM for longer than about 18 seconds when rehearsal is prevented.
Lab experiment means results are likely to be reliable.
Nonsense trigrams are artifical, so the study lacks ecological validity. Meaningful memories may last longer in STM.
Only one type of stimulus was used and the duration of STM may depend on the type of stimulus.
Each participant was shown different trigrams, which may cause confusion.
Bahrick et al (1975)
Aim: To study very long-term memories (VLTMs).
Method: 392 people asked to list names of ex-classmates (a 'free-recall test'). Shown photos and asked to recall names (photo-recognition test) or given names and asked to match them to a photo (name-recognition test).
Results: Within 15 years of leaving school, participants could recognise about 90% of names and faces. About 60% accurate on free recall. After 30 years, free recall declined to about 30% accuracy. After 48 years, name-recognition was about 80% accurate, photo-recognition about 40% accurate.
Conclusion: Shows evidence of VLTMs in 'real-life' settings. Recognition better than recall, so there may be a huge store of info, but it's not easy to access all of it.
High ecological validity.
Difficult to control all variables, which makes findings less reliable.
Shows better recall than other studies on LTM, which may be because meaningful info is stored better.
This type of info can be rehearsed, increasing the rate of recall, meaning results can't be generalised to other types of info held in LTM.
Aim: To study the capacity of STM.
Method: Participants shown a string of letters or digits. They had to repeat them back in the same order. The number of digits or letters increased until the participant failed to recall the sequence correctly.
Results: Participants mostly recalled about 9 digits and 7 letters. This capacity increased with age during childhood.
Conclusion: Jacobs concluded STM has a capacity of 5-9 items. Individual differences were found, such as STM increasing with age, possibly due to increased brain capacity or use of memory techniques, such as chunking. Digits may have been easier to recall as there were only 10 digits to remember, compared to 26 letters.
Artificial and lacks ecological validity
More meaningful information may be recalled better, perhaps showing STM to have an even greater capacity.
The previous sequences recalled by participants may have confused them in future trials.
Aim: To review research into the capacity of STM.
Results: Found people could remember seven items and argued the capacity of STM is seven, plus or minus two. He suggested using chunking to combine individual letters or numbers into larger more meaningful units.
Conclusion: Converting 2,0,0,3,1,9,8,7 into 2003 and 1987 makes the set of numbers easier to recall. STM could probably hold about seven pieces of chunked info, increasing its capacity.
Aim: To investigate encoding in STM and LTM.
Method: Participants given four sets of words either acoustically similar (e.g. man, mad, mat), acoustically dissimilar (e.g. pit, cow, bar), semantically similar (e.g. big, large, huge) and semantically dissimilar (e.g. good, hot, pig). Participants were asked to recall the words either immediately or following a 20-minute task.
Results: Participants struggled recalling acoustically similar words immediately (from STM). If recalling after an interval (from LTD), they had problems with semantically similar words.
Conclusion: LTM is more likely to rely on semantic encoding and STM on acoustic encoding.
Lacks ecological validity.
There are other types of LTM and other methods of encoding.
Used an independent groups design, so there wasn't any control over participant variables.
Atkinson and Shiffrin (1968) Multi-Store Model
1) Proposes that memory consists of three stores - a sensory store, a short-term store and a long-term store.
2) Info goes into sensory memory. Attention or thought allows this to become encoded and pass into STM.
3) Short-term memory has a finite capacity and duration. Info is processed further (rehearsed) and transfers to LTM.
The Primacy Effect shows participants are more able to recall items from the beginning of a list than the middle, as the info has been rehearsed and transferred to LTM.
The Recency Effect shows participants are more able to recall items at the end of a list as they are still in STM.
People with Korsakoff's Syndrome can recall the last items in a list, suggesting an unaffected STM, but struggle to recall items at the start, suggesting a damaged LTM.
People don't always spend time rehearsing in real life but info still gets to LTM.
The model is oversimplified.
Baddeley and Hitch (1974) Working Memory Model
The Working Memory Model suggests STM is made up of several different stores.
The Central Executive has a limited capacity and controls two slave systems:
1) The articulary-phonological loop holds speech-based info. It contains a phonological store (the inner ear) and and articulatory process (inner voice).
2) The visuo-spatial sketchpad deals with the temporary storage of visual and spatial info.
Baddeley and Hitch used studies of 'interference tasks' to make the model:
If participants are asked to perform two tasks simultaneously that use the same system, performance will be affected on one or both tasks, as the system has limited capacity. Performance is not affected if the tasks involve different systems.
Shallice & Warrington's study of KF (1974) suggested a damaged Articulatory Loop but an undamaged Visuo-Spatial Sketchpad.
The model has been criticised as simplistic and vague. It doesn't really explain what the Central Executive is, apart from being involved in attention.