An easy way to improve scoring of memory span tasks: The edit distance, beyond “correct recall in the correct serial position”

Abstract

For researchers and psychologists interested in estimating a subject's memory capacity, the current standard for scoring memory span tasks is the partial-credit method: subjects are credited with the number of stimuli that they manage to recall correctly in the correct serial position. A critical issue with this method, however, is that intrusions and omissions can radically change the scores depending on where they occur. For example, when recalling the sequence ABCDE, “ABCD” is worth 4 points but “BCDE” is worth 0 points. This paper presents an improved scoring method based on the edit distance, meaning the number of changes required to edit the recalled sequence into the target. Edit-distance scoring gives results close to partial-credit scoring, but without the corresponding vulnerability to positional shifts. A reanalysis of memory performance in two large datasets (N = 1093 and N = 758) confirms that in addition to being more logically consistent, edit-distance scoring demonstrates similar or better psychometric properties than partial-credit, with comparable validity, a small increase in reliability, and a substantial increase of test information (measurement precision in the context of item response theory). Test information was especially improved for harder items and for subjects with ability in the lower range, whose scores tend to be severely underestimated by partial-credit scoring. Code to compute edit-distance scores with various software is made available at https://osf.io/wdb83/.