Associative clustering may be defined as the occurrence of sequences of related words in the recall of a randomized stimulus word list. As indicated in an earlier study ( 3 ) , the operations for inducing and measuring clustering have typically involved the following steps: (a) the compiling of two or more sub-lists of equal lengths, of words with each sub-list representing words in a different taxonomic category; (b) the randomization of all the words thus chosen to form a stimulus list; (c) the presentation of the stimulus list for learning; (d) the obtaining of free recall; (e) the analysis of the recalled words to determine the incidence of clusters of two or more words falling in the same taxonomic category. The unit for the measurement of clustering is a repetition which is defined as a sequence of two words in the same operational category. In applying this measure, the number of repetitions in a cluster is designated as one less than the number of words in the cluster. The convention established in the original study of clustering ( I) was to rely upon E's judgment for the choice of words in the operational categories. To equate the response strengths of the words in the specified categories use was made of the Thorndike-Lorge (7) frequencies of usage. It was assumed that both the mean and the range of the frequencies of the words in each category should be similar. The feasibility of these operations was demonstrated in a study (2) employing two stimulus lists with the same categories in each. One comprised words with relatively high Thorndike-Lorge frequen.ties and the other words with relatively low Thorndike-Lorge frequencies. The data indicated a significant difference in both clustering and the number of words recalled in favor of the list with the higher frequencies. The present study was based on the choice of words selected from the