In a recent analysis of an internal SUS survey as SAP, it showed that adding 20-30 respondents to the existing 30, had less than 5% margin of error. 160 respondents or more reduce the margin of error to about 2.5 where it more or less stays even if hundreds more are added.This example shows that 30 well chosen respondents deliver a usable result.
“I usually feel like I tested too many users not too few. Unusable tasks reveal themselves pretty early on.”, Dean Barker, director of UX at Sage CRM, MeasuringU finds only 11% difference.