Chapter 7

What Sample Sizes Do We Need?

Part 2: Formative Studies

The purpose of formative user research is to discover and enumerate the events of interest in the study (e.g., the problems that participants experience during a formative usability study). The statistical methods used to estimate sample sizes for formative research draw upon techniques not routinely taught in introductory statistics classes. This chapter covers techniques based on the binomial probability model and discusses issues associated with violation of the assumptions of the binomial model when averaging probabilities across a set of problems or participants while maintaining a focus on practical aspects of iterative design.

sample size estimation, formative user research, ...

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