How many users do I need to run a SUS and get valid result?

Question

I ran a System Usability Scale survey on my team (7 people) but I'm not sure that's enough to get relevant result and I didn't find any article recommending anything about the number of people to survey.

Is 6-7 enough?

Answer 1

Measuring Usability has an article that briefly touches on sample size. TL;DR: 2 is the obvious minimum, 5 is a suitable starting point.

From 10 Things To Know About The System Usability Scale (SUS):

You can use SUS on small sample sizes: One common question I get when using the SUS (or when measuring usability in general) is about the lowest acceptable sample size. Technically you need at least two users to have some measure of variability (the standard deviation) and to generate confidence intervals. We have never done a test using the SUS with only two users. We will, however, report the SUS score with just five users.

Five is often a magic number for early-phase usability studies. Confidence intervals will be rather wide, but the average SUS score will be surprisingly stable. We did several computer simulations and showed that at a sample size of 5, the sample mean is within six points of a very large sample SUS score 50% of the time (see the graph below).

The figure above shows the difference between the average SUS score and a the mean from a sample size of just 5 repeated 1000 times. In 50% of the samples the SUS score from a sample size of 5 was within 6 points of the true SUS score. Not bad for such a small sample size.

In other words, if the actual SUS score was a 74, average SUS scores from five users will fall between 66 and 80 half of the time. Seventy-five percent of the time, the score differed by 10 points and 95% of the time, by about 17 points. In other words, you get within the ballpark of the actual SUS score in more than half of the cases with very small sample sizes. For more precise measures of sample sizes, use the SUS Guide and Calculator.