IMHO, that will depend on how you involve your participants and for what. E.g., if it is for assessing the occurrence of an issue, then it may be useful to have a look at http://www.measuringu.com/qualitative_sidebar.htm concerning the "magic number 5" and the usage of the formula behind it (as well as related pointers, e.g., how to estimate whether 5 is enough or not for the evaluation goal you've in mind). Cheers. Rosella

Hi Rita,

The Nielsen's argument to consider 5 users is based on the fact that you have a well defined target audience and that your focus is on qualitative results.

When you have similar goals and you are also considering accessibility, challenging contexts of use should be considered, for instance, different types of disability, multiples assistive technologies in place, and even different configurations/customizations of the assistive technologies in place.

In a recent text Nielsen presents that the best ROI comes when you consider 8 users: https://www.nngroup.com/articles/how-many-test-users/

In the same text, Nielsen says that for larger number of participants "Some design projects had multiple target audiences and the differences in expected (or at least suspected ) behaviors were large enough to justify the expense of sampling additional users"

This said, I've seen works presenting qualitative in depth studies involving specific target audiences where authors considered less than 5 people due to the difficulty of finding representative participants. Thus, if you are planning to consider a wide range of disabilities/capacities, I suggest you to consider few participants for each of the main characteristics of your target audience, this way you can cover the main accessibility barriers and/or usability problems. If you have a specific target audience, I suggest you to follow the approach presented by Nielsen, with multiple cycles of evaluation involving 5-8 participants.

Best regards,

Vagner

Hi Vagner and Rosella,

Thank you very much for your answers.

My research is qualitative and its target audience is very specific: TV viewers with visual impairment. In other similar researches that I carried out, I followed the approach presented by Nielsen (5-8 users) and it never desappointed me, but it's good to know other opinions and perspectives!

Best regards,

Rita

In the specific case of your target audience, I will suggest to take also in consideration how you approach your participants. Establishing a trust relationship is of paramount importance to get committed participants.

The participants in the evaluation/usability assessment should not be the same people who participated in the participatory design aspects of your study.  The design participants will know too much about the design being evaluated.  You need test participants who do not have prior knowledge of the design and how it is supposed to work.

Hi Rita!

A web article that don't give you a specific answer but have a list of the Pros and Cons. Reasons You Should and Should Not Test With 5 Users.

http://www.measuringu.com/blog/five-for-five.php

Best

Thanks Lígia! :D ;)

Thanks Christophe!