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I'm running a use test for 12 participants split into 2 sets of 3 subgroups (user type and user location - these are NOT mutually exclusive groups).

I need to produce 7 different SUS results: overall, u-type-a, u-type-b, u-type-c, geo-loc-a, geo-loc-b, geo-loc-c.

Obviously, SUS relies on self-reported statistics given after the experience - this means that the results are not necessarily 100% accurate and could probably benefit from being presented with some sort of margin for error (for error read 'misremembering', 'misunderstanding', 'alternate system bias', etc).

As the SUS is a relatively complex calculation there are plenty of points to add in an error margin: on the question scores, on the score total, on the percentage per user, or on the average for the user group.

My question is this: How much margin, if any, should I allow for and where in the calculation should I add it?

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My question is this: How much margin, if any, should I allow for and where in the calculation should I add it?

An error bar makes no sense for individual SUS scores.

Presenting grouped average scores then displaying a standard deviation for each group would be sensible.

If you're worried about the audience for the results reading too much into small variations of SUS scores between individuals or groups then maybe consider mapping the SUS scores onto an adjective rating scale.

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