Scaled “Ease of Remembrance” Values of Random Shapes and Stimulus Difficulty in Paired-Associate Learning

Attneave F. , & Arnoult M. D. The quantitative study of shape and pattern perception. Psychol. Bull., 1956, 53, 452–471.

Underwood B. J. , & Schulz R. W. Meaningfulness and verbal learning. Philadelphia: Lippincott, 1960.