A researcher conducts interviews with 15 female marathon first-time finishers to understand the psychological impact of the event on self-esteem. What type of design did this researcher most likely use?

case study.

Explanation:

A case study is a research method involving an up-close, intensive and detailed examination of a particular case in which the researcher examines in-depth data relating to several variables

The case study is not itself a research method, but researchers select methods of data collection and analysis that will generate material suitable for case studies.

In psychology, case studies are often confined to the study of a particular individual or a group of individuals. The information is mainly biographical and relates to events in the individual's past, as well as to significant events which are currently occurring in his or her everyday life.

to plan an idea out before

answer; procedural justice;