How many respondents are sufficient?”
Usability Tests geared towards a homogeneous target group merely require 10 to 12 participants to unearth the bulk of problems. If we already know in the run-up that the user group will consist of 2 dividable sub-groups, at least 16 respondents (a minimum of 8 per target group) will be necessary to reveal target-group-specific differences. In case special emphasis is given to the perception of the brand and user opinions concerning its design, we recommend to interview at least 20 respondents. To meet extreme demands, however, it may be useful to investigate up to 200 participants for a simultaneous testing of a multitude of services for a multitude of target groups. The selection of groups for a test should focus primarily on the question which distinct user scenarios you expect for your product rather than asking which market segments are targeted. If your budget or time merely allow of a small study, a test including 5 to 10 participants will certainly prove more efficient than none
