Surveys tell you what your customers do. In-depth interviews tell you why. For complex purchase decisions, sensitive subjects, and small or hard-to-reach audiences, that “why” is where the value lives — and it is the reason in-depth interviews have become one of the most-used qualitative methods in the industry.

Online in-depth interviews with webcams are now the single most common qualitative research method, used regularly by roughly a third of researchers worldwide. As demand for qualitative insight climbs and AI accelerates how fast transcripts can be analyzed, the bottleneck has shifted to a different part of the process: finding and confirming the right participants. A study is only as strong as the people in the room, and in 2026 that is harder to guarantee than it has ever been.

This guide covers what IDI market research is, how it differs from focus groups and surveys, what it costs, how many interviews you actually need, and why recruitment — not moderation — is the factor that most often determines whether the findings hold up.

What Is IDI Market Research?

In-depth interviews (IDIs) are one-on-one, semi-structured qualitative conversations between a trained moderator and a single participant. The moderator works from a discussion guide but follows the participant’s answers wherever they lead, probing for the reasoning, emotion, and context behind a decision rather than collecting a fixed set of responses.

That format makes IDIs the method of choice when the subject is complex, personal, or sensitive. A patient describing a treatment journey, an executive walking through a procurement process, or a shopper explaining the internal back-and-forth before a purchase will say more in a private one-on-one than in any group setting. There is no social pressure, no dominant voice steering the room, and no reason to perform for other participants.

IDIs sit alongside focus groups and surveys in the qualitative toolkit, and the modality has shifted decisively online. A research team can now run interviews across multiple time zones in a single week, with transcription and thematic analysis running in parallel. What has not changed is the dependence on getting the right person in front of the camera.

When to Use IDIs Instead of Focus Groups or Surveys

Choosing a method comes down to three questions: what are you trying to learn, who are you trying to reach, and how sensitive is the topic. IDIs win on all three when the answers point toward depth over breadth.

A useful rule: quantitative methods like surveys are built for measuring, and qualitative methods like IDIs are built for exploring. If you can already state your hypotheses and just need to size them, run a survey. If you are still trying to understand the decision itself, sit down with people one at a time.

How Many Interviews Do You Actually Need?

The honest answer is that it depends on what you mean by “enough,” and the research backs that up. The governing concept is saturation — the point at which new interviews stop producing new information.

A systematic review of empirical saturation studies found that most reached saturation at 9 to 17 interviews. A 2024 integrative review put theme saturation at around 9 interviews and meaning saturation — the deeper standard where you can explain the nuance and variability within a theme — at roughly 24. Theoretical saturation, the highest bar, often requires twice as many interviews as basic data saturation. The early empirical work that started this whole line of research found that the first five to six interviews tend to produce the majority of new information.

For most commercial projects, that lands somewhere between 8 and 24 interviews per audience segment, scaled up when the audience is diverse or the questions are exploratory. The exact number matters less than a more practical truth: every one of those interviews has to be filled by a qualified, verified participant, or the saturation point you reach is built on the wrong data.

What Do In-Depth Interviews Cost?

IDI costs are driven by a handful of factors that compound with the difficulty of the audience:

• Number of interviews. More sessions, higher cost. Niche or hard-to-reach segments raise the per-interview price.
• Length and complexity. Longer, more technical conversations demand more moderator time and expertise.
• Recruitment and incentives. Specialized audiences require more sourcing effort and larger honorariums to secure and confirm.
• Analysis and reporting. Transcription, coding, and synthesis carry their own cost, though AI tools are compressing the timeline.

Of these, recruitment is the line item that quietly determines whether the whole study is worth what you paid for it. Spend on moderation and analysis, and you still get nothing if the participants were not who they claimed to be.

Why Recruitment Determines the Reliability of IDI Research

This is where most studies are won or lost — and the risk has grown sharply. The shift of qualitative research online opened the door to a wave of fraudulent and low-quality participants, and the numbers are sobering.

Fraudulent respondents do not just add noise. They distort findings in consistent directions — overreporting things like excellent health and product ownership — and they have grown sophisticated enough to pass basic trap questions and IP checks. For a small IDI study where every interview carries real weight, even a few imposters can quietly invalidate the conclusions.

The defense is rigorous, human-led recruiting built on a verified participant base, not a scramble against the clock on an open exchange. That means:

This is also why recruitment for focus groups and IDIs is increasingly handled by specialist firms rather than absorbed into general panels. The methods overlap, but both depend on the same foundation: a confirmed, screened participant who is exactly who the study needs.

How an IDI Study Comes Together

1. Define objectives
Set clear goals and the criteria for who qualifies before any sourcing begins.

2. Recruit and screen
Source candidates against the criteria, verify identity, and confirm fit through live screening. For specialized audiences this is the longest and most decisive phase.

3. Conduct the interviews
A trained moderator runs each one-on-one session, probing for reasoning and context.

4. Analyze and report
Transcribe, code, and synthesize the conversations into themes and recommendations — increasingly with AI-assisted analysis to compress the timeline.

Sources

Backlinko — “23 Key Market Research Statistics for 2026,” January 2026
The Alchemic — “67 Market Research Statistics for 2026: AI, Growth & Trends,” January 2026
Statista — Most-used qualitative research methods, via Backlinko, 2026
Qualtrics — Demand for qualitative research and AI adoption, via Backlinko, 2026
Research & Metric — “Qualitative Research Trends Reveal Powerful Brand Truths,” March 2026
Greenbook — “The Pervasive Threat of Tech-Enabled Fraud in Survey Research,” August 2025
NORC at the University of Chicago — “The Fraud Problem Reshaping Survey Research,” February 2026
PLOS One — Guest, Namey & Chen, “A Simple Method to Assess and Report Thematic Saturation in Qualitative Research,” 2020
ScienceDirect — “Sample Sizes for Saturation in Qualitative Research: A Systematic Review of Empirical Tests,” 2021
SAGE — Wutich, Beresford & Bernard, “Sample Sizes for 10 Types of Qualitative Data Analysis,” 2024