Immediately after visual presentation, subjects were required to recall 6-letter sequences. Sequences were drawn from four vocabularies. There were two 3-letter vocabularies, distinguished by the probability of acoustic confusion within them, and two 9-letter vocabularies similarly distinguished. Memory span is shown to be effectively independent of information per item, and to depend substantially on the probability of acoustic confusion within vocabularies.A recent paper by the author (Conrad, 1964) demonstrated a highly significant association between errors in immediate recall of 6-letter sequences, and listening errors. Letters of the alphabet which are difficult to hear against a white noise background are also difficult to recall, even when presentation for recall is visual. The effect was also shown to hold true for words (Conrad, 1963).It was argued that, if letters which are acoustically confusing are more likely to be confused in memory, then sequences drawn from acoustically homogeneous vocabularies would be more difficult to recall than those drawn from vocabularies which were acoustically heterogeneous. It was suggested that the acoustic nature of the vocabulary might in fact be a more important variable in determining memory span than the size of vocabulary from which sequences were drawn. The present experiment tests these two hypotheses. METHODThirty-six 6-letter sequences were constructed from each of four consonant 'vocabularies '. The vocabularies were :The sequences were based on tables of random numbers with the following constraints : The vocabularies were chosen on the basis of two variables. First, vocabulary size, three or nine letters. Secondly, acoustic confusability within the vocabulary, so that for each size of vocabulary there was one which had high, and one which had low, confusability. This variable requires explanation.Conrad (1964) presents a confusion matrix for letters of the alphabet spoken against a white noise background. For subsets of this matrix composed of the letters of the

Results are presented which confirm data already published by Corballis (J 966). and Murray. (J 966J. These show that when a sequence of letters or digits is presented for immediate recall, there is a marked difference in the serial position curve of errors according to whether presentation is auditory or visual. This difference specifically shows as a virtual absence of recency in the visual presentation condition.

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A matrix is presented of the errors of perception made by 135 men and women listening to three male and three female speakers reading aloud different randomized lists constructed from the letters of the alphabet and the digits 1–9, heard in white noise. Data from a short‐term memory (STM) experiment, using simultaneous visual presentation and immediate ordered recall of two selected vocabularies of nine letters and the digits 1–9, are cited as evidence of phonemic confusion between letters and digits in STM.

Two measures which have been shown to predict the ease of learning trigrams, namely log letter frequency and sequential predictability, were applied to data from an experiment on short term memory. This involved the immediate recall of 120 six-letter consonant sequences which were presented visually one letter at a time. A significant correlation was found between the probability that a given sequence would be recalled correctly and both its mean log letter frequency (r = 0.308, p < 0.001), and its mean predictability (r = 0.393, p < 0.001). Partial correlation showed only a marginally significant effect of log letter frequency when predictability was partialled out (r = 0.161, 0.05 < p < 0.1). With log letter frequency partialled out, however, a reliable correlation between predictability and recall score remained (r = 0.300, p < 0.001).