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How To Specify the Participant Group Size for Usability Studies: A Practitioner’s Guide

Ritch Macefield

Journal of Usability Studies, Volume 5, Issue 1, Nov 2009, pp. 34 - 45

Article Contents

Punctuated Studies

For all of the types of usability studies discussed so far in this article, we can sometimes gain benefits by studying the participants in (predetermined) chunks punctuated by time slots for incrementally analyzing the data from each chunk. For example, suppose a study is concerned with problem discovery in a new interface design and we have specified a group size of 12 participants. We could break this group into three chunks of four participants and execute the study according to the following plan:

Suppose also that analysis of findings from chunk one reveals a severe problem with a new interface design. We could terminate the study at this point and possibly rectify the problem before continuing with the following chunks so that the interface design is evolved during its own study.

This tactic can lead to considerable cost and time savings but, perhaps more importantly, it is easy to see how with formative studies it promotes the idea of iterative design processes that are fundamental to a user-centered design (UCD) philosophy.

We can also gain benefits by designing punctuated studies with asymmetric chunks. This can be particularly useful in the case of comparative studies where we are seeking statistically significant findings. We could begin with a chunk of 10 participants (which may well meet our objectives) followed by three further chunks of five participants, which could be incrementally utilized if required.

In summary, although the logistics of punctuated studies are a little more complex and can potentially consume a little more resources than unpunctuated studies, punctuated studies have the potential to both significantly reduce the resources required for a study and promote a UCD approach. (It is worth noting here that usability agencies may be resistant to such designs because their businesses are easier to manage if studies have a fixed cost and duration.)


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