memory capacity duration

Research on coding

Once information gets into the memory system, it is stored in different formats, depending on the memory store. The process of converting information from one form to another is called coding. Alan Baddeley (1966a, 1966b) gave different lists of words to four groups of participants to remember:

• Group 1 (acoustically similar): words sounded similar (e.g. cat, cab, can).

• Group 2 (acoustically dissimilar): words sounded different (e.g. pit, few, cow).

• Group 3 (semantically similar): words with similar meanings (e.g. great, large, big).

• Group 4 (semantically dissimilar): words that all had different meanings (e.g. good, huge, hot).

Participants were shown the original words and asked to recall them in the correct order.

When they had to do this recall task immediately after hearing it (STM recall), they tended to do worse with acoustically similar words.

If participants were asked to recall the word list after a time interval of 20 minutes

(LTM recall), they did worse with the semantically similar words. This suggests that information is coded semantically in LTM.

Research on capacity

George Miller (1956) made observations of everyday practice. For example, he noted that things come in sevens; there are 7 notes on the musical scale, 7 days of the week, 7 deadly sins, and so on. This suggests that the span (or capacity) of STM is about 7 items (plus or minus 2). However, Miller also noted that people can recall 5 words as well as they can recall 5 letters. They do this by chunking - grouping sets of digits or letters into units or chunks.

Duration of LTM

Harry Bahrick and colleagues (1975) studied 392 participants from the American state of Ohio who were aged between 17 and 74. High school yearbooks were obtained from the participants or directly from some schools. Recall was tested in various ways, including: (1) photo-recognition test consisting of 50 photos, some from the participant's high school yearbook; (2) free recall test where participants recalled all the names of their graduating class.

Participants who were tested within 15 years of graduation were about 90% accurate in photo recognition. After 48 years, recall declined to about 70% for photo recognition.

Free recall was less good than recognition. After 15 years this was about 60% accurate, dropping to 30% after 48 years.

This shows that LTM can last a very long time indeed.

Short term memory

The limited-capacity memory store. Coding is mainly acoustic (sounds), capacity is between 5 and 9 items on average, duration is between about 18 and 30 seconds.

Long term capacity

The permanent memory store. Coding is mainly semantic (meaning), it has unlimited capacity and can store memories for up to a lifetime.

Baddeley Limitation

One limitation of Baddeley's study was that it used quite artificial stimuli rather than meaningful material. The word lists had no personal meaning to participants. This means we should be cautious about generalising the findings to different kinds of memory task. For example, when processing more meaningful information, people may use semantic coding even for STM tasks.

Miller Limitation

One limitation of Miller's research is that he may have overestimated the capacity of STM. For example, Cowan (2001) reviewed other research and concluded that the capacity of STM was only about four chunks.

This suggests that the lower end of Miller's estimate (five items) is more appropriate than seven items.

Strength Bahrick

One strength of Bahrick et al.'s study is that it has higher external validity. Real-life meaningful memories were studied. When studies on LTM have been conducted with meaningless pictures to be remembered, recall rates were lower (e.g. Shepard 1967).

The downside of such real-life research is that confounding variables are not controlled, such as the fact that Bahrick's participants may have looked at their yearbook photos and rehearsed their memory over the years.

Duration of STM

Duration is the defining feature of STM. But how short is it exactly? Margaret and Lloyd Peterson (1959) tested 24 undergraduate students. Each student took part in eight trials.

A 'trial' is one test. On each trial the student was given a consonant syllable (also known as a trigram, such as YCG) to remember and was also given a 3-digit number. The student was then asked to count backwards from that 3-digit number until told to stop.

This counting backwards was to prevent any mental rehearsal of the consonant syllable (which would increase the student's memory for the consonant syllable).

On each trial they were told to stop after a different amount of time - 3, 6, 9, 12, 15 or 18 seconds. This is called the retention interval. Their findings are shown in the graph on the left. It suggests that STM may have a very short duration indeed, unless we repeat something over and over again (i.e. verbal rehearsal).

Limitation Peterson

A limitation of Peterson and Peterson's study is that the stimulus material was artificial. Trying to memorise consonant syllables does not reflect most real-life memory activities where what we are trying to remember is meaningful. So we might say that this study lacked external validity