In-Depth Interviews as a Tool for Qualitative Research: Theoretical Foundations, Typologies, Design, Analysis, and Emerging Trends

Abstract

The in-depth interview (IDI) is the most widely used method in qualitative research, offering unparalleled access to the subjective meanings, lived experiences, and interpretive frameworks through which individuals construct and narrate their social worlds. This paper provides a comprehensive and critically grounded methodological review of IDIs as a qualitative research tool, tracing their intellectual lineage from psychoanalytic case-study practice and anthropological fieldwork through the formal codification of semi-structured and unstructured interviewing in the sociology of the 1960s and 1970s to their contemporary deployment in online, AI-assisted, and mixed-methods contexts. The paper systematically addresses the definitional boundaries and epistemological foundations of the IDI; a typology of eight format variants; principles of purposive sampling and theoretical saturation; the architecture of the interview guide; the craft of probing and active listening; the management of power, positionality, and ethics; data collection and transcription practice; and six major analytical frameworks   thematic analysis, interpretive phenomenological analysis, grounded theory, narrative analysis, framework analysis, and discourse analysis. Six structured tables consolidate comparative and decision-making guidance. The advantages of IDIs   depth, flexibility, privacy, rapport, and analytical versatility   are evaluated against their limitations, including cost, interviewer effects, social-desirability bias, and the demands of reflexivity. Recent developments, including the validation of online video interviews, AI-assisted transcription, and the use of large language models for first-pass coding, are assessed critically. The paper concludes that the IDI remains methodologically indispensable wherever depth, nuance, and the individual's own interpretive voice are the primary objects of inquiry   provided the method is implemented with epistemological self-awareness, ethical rigour, and transparent analytical practice.