A person’s span of working memory is the theoretical maximum number of items (“Chunks” in human cognition) they can remember simultaneously, without committing any to long-term memory.
==TODO: I’m conflating “immediate memory” and “working memory” here, without fully understanding the relationship. Some researchers seem to treat these terms interchangeably, while others (Conway, 2005, p. 779) draw an intentional distinction I haven’t yet absorbed.==
It’s not a precisely measurable quantity: the definition of an item is fuzzy, and subjects’ spans vary somewhat depending on context and item type. Usually experimenters will talk about a more specific working memory span, like “reading span,” which refers to a task in which {subjects read a sequence of sentences and try to remember the last word in each} (see review of task types, Conway et al, 2005). ☉
While the value does vary by task and context, it appears to be surprisingly consistent. There’s considerable debate among cognitive psychologists about the mean. Early-to-mid-20th century experimenters (e.g. Crannell and Parrish, 1957) put the value around 5-7, but more recent experimenters suggest (e.g. Cowan, 2001) that the value is closer to {4}, and that higher values indicated subjects subtly recoding stimuli into larger chunks (see Human channel capacity increases with bits-per-chunk). ☉
Related: Complex ideas may be hard to learn in part because their components overflow working memory
Conway, A. R. A., Kane, M. J., Bunting, M. F., Hambrick, D. Z., Wilhelm, O., & Engle, R. W. (2005). Working memory span tasks: A methodological review and user’s guide. Psychonomic Bulletin & Review, 12(5), 769–786. https://doi.org/10.3758/BF03196772
Cowan, N. (2001). The magical number 4 in short-term memory: A reconsideration of mental storage capacity. Behavioral and Brain Sciences, 24(1), 87–114. https://doi.org/10.1017/S0140525X01003922
Crannell, C. W., & Parrish, J. M. (1957). A Comparison of Immediate Memory Span for Digits, Letters, and Words. The Journal of Psychology, 44(2), 319–327. https://doi.org/10.1080/00223980.1957.9713089