Final answer:
The key criterion for being included in a sample for a phenomenological study is willingness to participate.
Step-by-step explanation:
In a phenomenological study, a key criterion for being included in a sample is willingness to participate. Phenomenologists seek to understand the essence and meaning of a phenomenon as it is experienced by the researcher(s). Thus, having participants who are willing to actively engage and share their experiences is crucial to gathering the necessary data.