Keywords: patient reported outcome, PRO, validation, saturation, FDA PRO guidance, capture recapture
PRO's developed de novo, using the 2009 FDA guidance may involve structured patient interviews or focus groups. Qualitative Research is a methodology for coding interviews and produces concepts or themes. These concepts are used to develop items in a PRO. A convention in the field is that interviews and code/concept elicitation are considered complete when subsequent interviews produces "no new concepts" -termed "saturation". The convention in the field is that there are sufficient patient interviews when the first occurrence of an interview with zero new concepts. Several authors have reported that this definition is inadequate and that concrete criteria are needed for confirming that saturation is achieved (Francis 2010, Mason 2010, Marshall 2013). Statistical methods are introduced for prospectively estimating sample size (number of interviews), defining saturation on a probability scale, and Type I error in the saturation process. Capture-recapture methodology is suggested for estimating the number of concepts as early as the second or third interview. On-going Research implements a Bayesian method suitable for small sample sizes.