How do you calculate the cognitive memory load requirements of an interface?

Question

If you break an interface down into elements and possible actions, how do you calculate the memory load requirements of an interface? Further, how do you relate the existing familiarity of an element and/or actions? What effect does conflict with an existing model have on memory load requirements?

UPDATE: The above is a reference to Cognitive modeling methods, and effective working memory capacity. For example, the page states that "for a younger adult, reasonable estimates of effective working memory capacity are 5-9 items. Long-term memory is believed to have an infinite capacity and decay time."

Answer 1

You can't calculate the cognitive memory load of an interface, but you can calculate (estimate) the cognitive memory load of performing a particular task using an interface.

I don't have extensive knowledge of the "state of the art" when it comes to cognitive modeling, but I have done some GOMS modeling in the past. The general technique is to break down the task into basic steps (i.e., "keystroke-level" tasks), such as "point mouse at item on screen", "read value", "remember value", "type a character", etc., assign estimated times (from the literature) to each step, then add them up to get an estimate for the time to complete the task.

Since you're interested in memory in particular, you can just examine the steps that involve memory. Examples include "recall X chunks* of data from long-term memory", "recall X chunks of data from short-term memory", "store X chunks of data in working (short-term) memory", etc.

Once you've done this, there are several analyses you could do. Examples:

1. How many memory operations are required to perform this task?

2. How many memory overloads are likely to occur? Does the user ever need to accumulate more then 5 chunks of data in working memory to perform the task? If so, how often?

3. How much time must be spent on memory operations? A very rough time estimate would be 1.2 seconds per memory operation per chunk. So, accessing 3 chunks of data from long-term memory would require 3.6 seconds. Same with working memory.

*Note: "chunk" is really just a fancy way of saying "thing", as in "people can remember 7 plus or minus 2 things". The point is that each chunk takes up one "slot" in your memory store, so you might be able to remember a 10-digit phone number because you are able to process the first three numbers (the area code) as a single "chunk".

You might find this link helpful: Keystroke-Level Model

Answer 2

Looks like a lot of folks misunderstood your question: Cognitive Workload, or Mental Workload is a well-defined term in psychology that can be used to compare one interface to another and detect where the pain points are in a particular task. There are several ways it can be measured, evaluated, and calibrated.

In my dissertation (tl;dr version), I measured mental workload in personal information management tasks such as managing files, calendars and contacts across multiple devices. Depending upon how much time & budget you have, you can either use the NASA Task Load Index, a subjective evaluation technique, or eye-tracker based cognitive workload measurement. Note that the eye-tracker isn't used to track the eye, but to measure the radius of the user's pupil, since a lot of biometric studies have shown correlation between a user's pupil size and mental workload, if other factors such as illumination are kept constant.

Details of both are available in the dissertation as well as on my site, if you're interested. E.g. I found that cognitive workload increases significantly at the point where you ask users to transition their work from one device to another, and in some cases, it is never restored back to the pre-transition levels.

Answer 3

You're referring to cognitive capacity...

http://en.wikipedia.org/wiki/The_Magical_Number_Seven,_Plus_or_Minus_Two