Coding, Capacity and Duration of Memory

Question 1

Key terms:

Answer 1

Coding = The format in which information is stored in the various memory stores.
Capacity = The amount of information that can be held in a memory store.
Duration = The length of time information can be held in memory.
Short-term memory (STM) = The limited-capacity memory store. Coding is mainly acoustic (sounds), capacity is between 5-9 items on average, and duration is between 18-30 seconds.
Long-term memory (LTM) = The permanent memory store. Coding is mainly semantic (meaning), it has unlimited capacity and can store memories for up to a lifetime.
Acoustically = Refers to sounds or the sense of hearing
Semantically = The meaning of something, such as a word.
Digit span = A way of measuring the capacity of STM in terms of the maximum number of digits that can be recalled in the correct order.
External validity = The degree to which a research finding can be generalised to, for example, other settings (ecological validity), other groups of people (population validity) and over time (historical validity).

Question 2

Research on coding

Answer 2

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.
Baddeley (1966a/b) gave different lists of words to 4 groups of ppts to remember:
Group 1 (acoustically similar): words that sounded similar (e.g. cat, cab, can).
Group 2 (acoustically dissimilar): words that 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).
Ppts 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 ppts were asked to recall the word list after a time interval of 20 mins (LTM), they did worse with the semantically similar words. This suggests that information is coded semantically in LTM.

Question 3

Evaluation of research on coding

Answer 3

• Baddeley’s study used artificial stimuli rather than meaningful material. The word list had no personal meaning to ppts which means we should be cautious when generalising the findings to different kinds of memory tasks. For example, when processing more meaningful info, people may use semantic coding even for STM tasks. This suggests that the findings from this study have limited application.

Question 4

Research on capacity

Answer 4

Digit span
Jacobs (1887) developed technique to measure digit span. The researcher gives, for example, 4 digits and then the ppt is asked to recall these in the correct order out loud. If this is correct the researcher reads out 5 digits and so on until the ppt cannot recall the order correctly. This determines the individual’s digit span. Jacobs found that the mean span for digits across all ppts was 9.3 items. The mean span for letters was 7.3.

Span of memory and chunking
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/capacity of STM is about 7 items (+/- 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.

Question 5

Evaluation of research on capacity

Answer 5

• Jacobs’ study was conducted a long time ago. Early research often lacked adequate control. For example, some ppts may have been distracted while they were being tested so they didn’t perform as well. This would mean that the results might not be valid because there were confounding variables that weren’t controlled. However, the results of this study have been confirmed in other research, supporting its validity.
• Miller may have overestimated the capacity of STM. For example, Cowan (2001) reviewed other research and concluded that the capacity of STM was only about 4 chunks. This suggests that the lower end of Miller’s estimate (5 items) is more appropriate than 7 items.

Question 6

Research on duration

Answer 6

Duration of STM
Peterson & Peterson (1959) tested 24 undergraduate students. Each student took part in 8 trials. On each trial/test the student was given a consonant syllable (aka a triagram, 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 of it). On each trial they were told to stop after different amounts of time - 3, 6, 9, 12, 15 or 18 secs. This is called the retention interval. Their finding show how the longer the retention interval, results in a lower % of correct responses (e.g. a negative correlation). It suggests that STM may have a very short duration indeed, unless we repeat something over and over again (i.e. verbal rehearsal).

Duration of LTM
Bahrick et al. (1975) studied 392 ppts from the American state of Ohio who were aged between 17-74. High school yearbooks were obtained from the ppts or directly from some schools. Recall was tested in various ways, including:
1) Photo-recognition test consisting of 50 photos, some from the ppt’s high school yearbook.
2) Free recall test where ppts recalled all the names of their graduating class.
Ppts 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 tim indeed.

Question 7

Evaluation of research on duration

Answer 7

• Peterson & Peterson’s stimulus material was artificial. Trying to memorise consonant syllables does not reflect real-life memory activities where what we’re trying to remember is meaningful. So we might say that this study lacks external validity.
  + Bahrick et al.’s study has higher external validity because real-life meaningful memories were studied. When studies 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 aren’t controlled - such as the fact that the ppts may have looked at their yearbook photos and rehearsed their memory over the years.