Introduction to Baddeley's study

Baddeley's 1966b study is a classic investigation in cognitive psychology that explores how information is encoded in memory. Encoding refers to the process of converting sensory input into a form that can be stored and retrieved later. The study specifically examines differences between short-term memory (STM) - a temporary storage system holding information for brief periods, typically up to 30 seconds - and long-term memory (LTM) - a more permanent storage system capable of holding vast amounts of information over extended periods.

Aims of the study

The primary goal was to determine how acoustic similarity - words that sound alike, such as "man" and "can" - and semantic similarity - words with related meanings, such as "great" and "large" - affect the learning and recall of word sequences in STM and LTM.

Key objectives

• To test if acoustic similarity disrupts recall more in STM, suggesting STM relies mainly on acoustic coding.
• To examine if semantic similarity impairs recall more in LTM, indicating LTM primarily uses semantic coding.
• To compare performance against control lists of dissimilar words to establish baseline effects.

If participants struggled with word order recall due to similarity, it would imply that the type of similarity confuses the encoding process in that memory system.

Participants

The study involved 72 male and female volunteers recruited from the Applied Psychology research unit at Baddeley's university.

Participants were divided into four independent groups, with 15-20 individuals per condition (15 in the acoustically similar group and 16 in the semantically similar group). This independent groups design ensured each participant experienced only one word list condition.

Method

Baddeley conducted a controlled laboratory experiment to systematically test memory recall under different conditions. This approach allowed precise manipulation of variables while minimising external influences.

Experimental design features

• Independent variable - Type of word list: acoustically similar, acoustically dissimilar (control), semantically similar, or semantically dissimilar (control).
• Dependent variable - Accuracy of recalling the word sequence in the correct order, measured during initial trials (testing STM) and a surprise retest (testing LTM).
• Control measures - Interference tasks were used to prevent rehearsal, ensuring the study isolated STM and LTM effects. Word lists were matched for frequency of everyday use to avoid confounding variables like familiarity.

Four lists of ten words each were prepared:

• List A: Acoustically similar words (e.g., man, can, cat, map).
• List B: Acoustically dissimilar words, matched to List A for frequency (e.g., pit, few, cow, mat) - served as control.
• List C: Semantically similar words (e.g., great, large, big, broad).
• List D: Semantically dissimilar words, matched to List C for frequency (e.g., good, old, deep, late) - served as control.

Procedure

The experiment followed a structured sequence to present words, introduce interference, and test recall. This ensured consistent testing across groups and focused on sequence order rather than word memorisation itself.

Step-by-step process

1. Word presentation - Participants viewed their assigned list of ten words projected one at a time, every three seconds, in a fixed order.
2. Interference task for STM - Immediately after presentation, participants completed a brief task: writing down eight numbers presented at one-second intervals. This blocked rehearsal and isolated STM effects.
3. Initial recall - Participants had one minute to write the word sequence in the correct order. To aid focus on order (not word learning), the words were visible on a card in random order.
4. Repetition - Steps 1-3 were repeated over four learning trials to build familiarity.
5. Longer interference for LTM - After the trials, participants copied eight-digit sequences at their own pace for 15 minutes to clear STM and test LTM.
6. Surprise retest - Without warning, participants recalled the word sequence again, assessing LTM retention.

This procedure distinguished STM (initial trials) from LTM (retest) by using timed interference to prevent information transfer between systems.

Results

The study measured recall accuracy across conditions, revealing distinct patterns for STM and LTM. Results focused on how similarity affected sequence order recall compared to controls.

Acoustic similarity effects

• In initial trials (STM), recall was poorer for acoustically similar words (List A) than dissimilar controls (List B), indicating confusion from sound-based coding.
• At retest (LTM), acoustic similarity had no significant impact, with recall similar between List A and List B.

Semantic similarity effects

• In initial trials (STM), semantic similarity (List C) did not notably affect recall compared to controls (List D).
• At retest (LTM), recall was much worse for semantically similar words, showing interference from meaning-based coding.

STM was disrupted by acoustic similarity, while LTM was impaired by semantic similarity. Control lists provided baselines, confirming these effects were due to similarity rather than other factors.

Conclusions

Baddeley's findings provide evidence for different encoding processes in STM and LTM, contributing to multi-store models of memory.

Main implications

• STM primarily encodes information acoustically, as shown by difficulties with sound-alike words in early recall.
• LTM mainly uses semantic encoding, evidenced by poorer recall of meaning-related words in the retest.
• Encoding is not exclusive; while LTM is primarily semantic, other forms may play minor roles.
• The study supports the idea that memory systems operate differently, with interference tasks confirming separation between STM and LTM.