Cognitive Task Analysis (CTA) helps instructional designers understand what cognitive activities required to perform a certain expertise. It appears to be a highly rigorous method to systemically elicit expert cognitive processes and decisions that other analysis methods cannot capture. The purpose of this study is to explore the qualitative changes on the vocalized data produced by CTA, through an observation and at least more than three or four time of interviews with three individual experts and provide implementation principles for CTA. The task for CTA is to convert an educational video for mobile service. Criteria of experts`` performance will be elicited with a subject-matter expert with 10 years of experience after recording the process of converting a video. Then, CTA interviews will be conducted twice each on three experts with more than five years of experience, and no experience of teaching. Concept, Principle, and Process (CPP) method and Critical Decision Method (CDM) will be used for the interviews. The vocalized data will be analyzed how the information from the interviews is qualitatively varied. It is expected that the results provide a practical guideline for conducting CTA.