Peterson and Peterson - STM Using Trigrams
Method: Participants where shown nonsense trigrams and asked to recall them after either 3, 6, 9, 12, 15 or 18 seconds. During the pause they were asked to count backwards in threes from a given number. This was an interface task- prevented from repeating the letters to themselves.
Results: After 3 seconds, participants could recall about 80% of trigrams correctly. After 18 seconds, only abot 10% were recalled correctly.
Conclusion: When rehearsal is prevented, very little can stay in STM for longer than 18 seconds.
Evaluation: These results are likely to be reliable- it's a laboratory experiment where the variables can be tightly controlled. However, nonsense trigrams are artificial, so the study lacks ecological validity. Meaningful or real-life memories may last longer in STM. Only one type of stimulus was used- the duration of STM may depend on the type of stimulus. Each participant saw many different trigrams. This could have led to confusion, meaning that the first trigram was the only realistic trial.
Bahrick et al - LTM in a Natural Setting
Method: 392 people were asked to list the names of their ex-classmates (free-recall). They were then shown photos and asked to recall the names of the people shown (photo-recognition) or given names and asked to match them to a photo of the classmate (name-recognition)
Results: Within 15 years of leaving school, participants could recognise about 90% of names and faces. They were about 60% accurate on free recall. After 30 years, free recall had declined to about 30% accuracy. After 48 years, name-recognition was about 80% accurate and photo-recognition about 40% accurate.
Conclusion: The study shows evidence of VLTMs* in a real life setting. Recognition is better than recall, so there may be a huge store of information, but it's not always easy to access all of it.
Evaluation: This was a field experiment so had high ecological validity. However, hard to contol all the variables, making findings less reliable. It showed better recall than other studies on LTM, may be because meaningful information is stored better. This type of information could be reheared, increasing rate of recall. This means results can't be generalised to other typed of information held in LTM.
* Very long-term memories
Jacobs - The Capacity of STM
Method: Paticipants were presented with 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: The majority of the time, participants recalled about 9 digits and about 7 letters. This capacity increased with age during childhood.
Conclusion: Based on the range of results, Jacobs concluded that STM has a limited storage capacity of 5-9 items. Individual differences were found, such as STM increasing with age, 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 different digits to remember, compared to 26 letters.
Evaluation: Jacobs' research is artificial and lacks ecological validity- it's not something you'd do in real life. More meaningful information may be recalled better, perhaps showing STM to have an even greater capacity. Also the previous sequences recalled by participants might have confused them on future trials.
Baddeley - Encoding in STM and LTM
Method: Participants were given 4 sets of words that were either acoustically similar (e.g man, mad, mat), acoustically dissimilar (e.g pit, cow, bar), sematically similar (e.g big, large, huge) and semantically dissimilar (e.g good, hot, pig). The experiment used an independent groups design- participants asked to recall the words either immediately or following a 20 minute task.
Results: Participants had problems recalluing acoustically similar words when recalling the word list immediately (from STM). If recalling after an interval (from LTM), they had problems with semantically similar words.
Conclusion: The patterns of confusion between similar words suggest that LTM is more likely to rely on semantic encoding and STM on acoustic encoding.
Evaluation: This study lacks ecological validity. Also, there are other types of LTM (e.g episodic memory, procedural memory) and other methods of encoding (e.g visual) which this experiment doesn't consider. The experiment used an independent groups design, so there wasn't any cotrol over participant variables.