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