Coding, capacity and duration of memory

Capacity

The amount of information that can be held in a memory store

Chunking

Grouping sets of digits or letters into chunks

Coding

The format in which information is stored in the various memory stores

Duration

The length of time information can be held in memory.

Long-term memory

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

Short-term memory

The limited-capacity memory store. In STM coding is mainly acoustic (sounds), capacity is between 5 and 9 items and duration is about 18 seconds

Alan Baddeley 1996

Researched coding

Gave different lists of words to 4 groups of participants to remember:

1. Acoustically similar words (sound similar)

2. Acoustically dissimilar words (sound different)

3. Semantically similar (similar meaning)

4. Semantically dissimilar (different meanings)

Participants were shown these words and were asked to recall them in order. They started by recalling immediately from their STM and did worse with acoustically similar words. After 20 minutes they recalled from their LTM and did worse with semantically similar words.

Suggests that information is coded acoustically in STM and semantically in LTM.

Jacobs 1887

Wanted to find out how much information could STM hold at a time.

Researched digit span of coding

For example a researcher reads out 4 digits and the participant can recall them all correctly.

If this is correct the researcher can read out 5 digits and increase the number until the participant can’t recall them anymore.

This is the individual’s digit span.

He found that the mean span across all participants for digits was 9.3. The mean span for letters was 7.3

Miller 1956

Made observations for everyday practice.

He noted that everything comes in 7s (7 days a week, 7 notes in a scale)

He thought this wasn’t an accident and suggested that the capacity of STM might be about 7 plus or minus 2.

Also noted that people can recall 5 words as easily as they can recall 5 letters. Doing this by chunking

Peterson and Peterson 1959

Tested 24 students in 8 different trials.

On each trial the student was given a trigram or a nonsense syllable (XYG) to remember. They also were given a 3 digit number to count backwards from. Prevents any mental rehearsal.

They were told to stop after varying periods of time, 3, 6, 9,12, 15, 18 seconds.

After 3 seconds 80% could recall it correctly. After 18 seconds only 3% could recall correctly.

They suggested that STM duration may be about 18 seconds unless we repeat the information over and over.

Bahrick 1975

Studied 392 American participants aged between 17 and 74. High school yearbooks were obtained from the participants or directly from their schools.

They tested recall in different ways:

1. Photo recognition test (50 photos) some from the yearbook.

2. Free recall where participants recalled all names of their graduating class.

Participants tested within 15 years of graduation were accurate 90% of the time.

After 48 years recall dropped to 70%

Wagenaar study

Recorded 2400 events over 6 years in a diary and tried to recall them

Very accurate recall, supports the fact that the capicity is extremely large.

Strength of Baddeley’s study

Identified a clear difference between 2 memory stores.

His findings of STM being acoustic and LTM being semantic have stood for a long time despite later research showing exceptions to his findings.

Led to the multi-store model

Baddeley artificial stimuli

The word lists had no personal meaning to participants

Findings may not tell us much about coding in different kinds of memory tasks especially in everyday life.

People may semantically code in STM when processing more meaningful information

Limited application

Strength of Jacobs’ study

Has been replicated.

Old studies lack adequate controls but replications can add these controls.

For example some participant’s digit spans might be underestimated because they were distracted while being tested (extraneous variables)

Jacobs’ findings were confirmed by other studies which were more controlled.

Shows it is a valid test of digit span in STM

Limitation of Miller study

May have overestimated STM capacity

Nelson Cowan 2001 reviewed other research and concluded that the capacity of STM is 4 + or - 1 chunks.

This suggests that the lower end of Miller’s estimate (5) is more appropriate than 7 items

Limitation of Peterson and Peterson

The results may be because of interference and not decay

They concluded that forgetting in STM was due to the decay of memory traces over time. However, the interference from counting backwards could be the reason

Their conclusions about STM duration may not be valid

Strength of Bahrick study

High external validity

Researchers investigated meaningful memories. When studies on LTM were conducted with meaningless pictures to be remembered the recall rates were lower.

Bahrick’s findings reflect a more real estimate of the duration of LTM