Sample sizes for usability tests: mostly math, not magic

@article{Lewis2006SampleSF,
  title={Sample sizes for usability tests: mostly math, not magic},
  author={James R. Lewis},
  journal={Interactions},
  year={2006},
  volume={13},
  pages={29-33}
}
"Really, how many users do you need to test? Three answers, all different."---User Experience, Vol. 4, Issue 4, 2005 

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