UX practitioners often use the term Cognitive Load to describe something bad. User have difficulties processing all information required to complete a task, and may fail, falls into the category: Cognitive Load.

It's nice that we have a common phrase for it, but do we agree on its impact? Is it possible to measure cognitive load, and if so, what is the scale? Can we quantify Cognitive Load?

  • 1
    This will be much better answered on CogSci.SE... Aug 3 '14 at 5:26
  • This would presumably be characterised by dropping out, taking a long time, scrolling or navigating back and forth. So you may be able to measure indirectly. There is likely a couple of underlying causes - processing time for complex inputs, and more importantly the limitations of working memory. Working memory is generally accepted to be 7+-2 items though it's recently been suggested it's lower.
    – Andy Boura
    Aug 3 '14 at 6:23
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    Cog.Sci. may have an answer that's scientifically sound, but UX is more of an engineering discipline. We'd want a way to measure Cognitive Load that doesn't involve electrodes, for instance.
    – MSalters
    Aug 3 '14 at 23:16
  • @MSalters Speaking as a cognitive psychologist I can assure you that if there's a non-invasive way to measure cognitive performance, CogSci people will be much more familiar with it than UX people. Aug 4 '14 at 5:46
  • This may be slightly pedantic but may help clarify the question: is this about predicting cognitive load rather than measuring it? Or maybe measuring how much cognitive load an interface is likely to cause. (Cognitive load is a property of people but we're really interesting in measuring our designs, maybe, I think...)
    – edeverett
    Aug 4 '14 at 13:03

Card, Moran and Newell’s Goals, Operators, Methods, and Selection (CMN-GOMS) family of models can be used to represent cognitive load. Many are familiar with the KLM-GOMS version, which models the physical steps in a task, but there are also elements and versions of GOMS (e.g., CPM-GOMS) which seek to model the cognitive steps in a task. The raw number of steps, the time to complete the cognitive steps (allowing for parallelism), and the number of levels in a task tree each represent different aspects of cognitive load.

While KLM-GOMS is relatively easily applied with little training, other flavors of GOMS take more expertise and effort to apply, which may account for them not being used as much. Instead of measuring and comparing cognitive loads, it’s easier to analyze a task to identify what cognitive load can be shifted to the computer (e.g., the current page replicates information from another page to reduce memory burden; the page computes and displays differences and percents to reduce mental arithmetic). In this way, we approach the minimal feasible cognitive effort without actually measuring how much it is.

As for the impact of cognitive load, usability seeks to minimize the time, effort, and errors in using a product. Each cognitive step you impose on a user represents a little more time, a little more effort, and a little more chance of error. Shifting these steps to the product UI improves usability as long as the product does it faster and more accurately than the user.


One accepted way to measure the cognitive load would be to apply Hick's law which

describes the time it takes for a person to make a decision as a result of the possible choices he or she has: increasing the number of choices will increase the decision time logarithmically

  • +1 Nice! I didn't know this! Welcome to UX.SE sazemaster. Aug 3 '14 at 14:52
  • Now we only need to define what at "choice" is. Spontaneously I would say every possible interaction at a specific point is a "choice". So when you can scroll up and down, those are already two "choices" and so on... Aug 5 '14 at 10:15

Rather a quick and dirty method is just to look at hesitations in speech (which obviously one can count up from a recording). People's ability to conduct a conversation tends to degrade with other sources of mental loading.

  • Unforunately, everything "new" requires an immense amount of CL. So when exposing test subjects to a new interface, they will have a hard time talking and testing simultaneously. When I started working in IxD, I also said to subjects to "think loud". However they tend to either concentrate to much on talking instead of interacting or interacted and didn't talk. I now don't encourage anymore subjects to think loud. Instead I observe them, make notes and ask them right after the test. Most of the time, they remember the questionable issue and give me good answers to work with. Aug 5 '14 at 10:22

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