The Wigmore criteria:

• the communications must originate in a confidence that they will not be disclosed.
• this element of confidentiality must be essential to the full and satisfactory maintenance of the relation between the parties.
• the relation must be one which in the opinion of the community ought to be sedulously fostered.
• the injury that would inure to the relation by the disclosure of the communications must be greater than the benefit thereby gained for the correct disposal of litigation.

(Wigmore, 1905; italics in original)

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See Palys & Lowman (2000) for consideration of how the Wigmore criteria apply to the researcher-participant relationship.

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