Research methods
In-depth interviews (IDIs): when to use them in market research and how to run them
What in-depth interviews are, when to choose them over focus groups or surveys, how many to run, and how to write the guide, run each session and report results.
An in-depth interview (IDI) is a one-to-one research conversation, usually lasting 30 minutes to two hours, in which an interviewer explores a person's experiences, decisions and reasons in detail. Pathfinder International's guide to the method describes it as intensive individual interviews with a small number of respondents to explore their perspectives on a particular idea, program or situation (Pathfinder International). In market research, IDIs are a common way to find out why people buy, switch or stop, before you know the reasons well enough to count them in a survey.
Most IDIs are semi-structured. The interviewer works from a guide of topics and questions but can change the order, skip questions and follow up on anything unexpected. Nielsen Norman Group notes that the semistructured interviews UX researchers run are what market researchers call depth interviews (Nielsen Norman Group).
When to use in-depth interviews
Pathfinder's guide says IDIs are useful when you want detailed information about a person's thoughts and behaviors or want to explore new issues in depth. They are often used to put other data in context, such as figures showing that something changed, and to refine the questions for a later survey.
| Method | Choose it when | Watch out for |
|---|---|---|
| In-depth interviews | You need each person's reasons and the sequence of a decision | Small samples; time to run and analyze |
| Focus groups | You want reactions to a concept, name or message, and discussion between people helps | Group influence; weak for detailed personal behavior |
| Survey | You know the likely answers and need to count how common each is | Can't explain answers or follow up |
Pathfinder also recommends individual interviews over focus groups when people may not be comfortable talking openly in a group, or when you need individual rather than group opinions. That covers a lot of B2B research, where buyers may not want to discuss vendors, budgets or internal decisions in front of people from other companies. Focus group research covers when the group format is worth it.
A worked example
This example is hypothetical. A payroll software company sees in its data that restaurant groups with 5 to 20 locations cancel at a higher rate than its other customers, usually in their first year. The data shows when they leave. It doesn't show why, or what they switch to.
The team plans IDIs with three groups:
- Finance managers at restaurant groups that cancelled in the past six months
- Finance managers at similar groups that stayed
- A few people who sell payroll software to restaurants and have seen many switching decisions from the other side
Each interview will run 45 minutes by video.
How many interviews
There is no fixed number. A 2022 systematic review of studies that tested empirically when interviews stopped producing new findings put that point at 9 to 17 interviews, and noted that most of those studies had fairly similar participants and narrowly defined objectives (Hennink and Kaiser, Social Science & Medicine). Pathfinder's rule of thumb points the same way: you have done enough when you keep hearing the same stories, themes and issues.
Plan per group rather than per study. For the payroll company, that means starting with a handful of leavers and a handful of stayers, with room to add more if their reasons keep varying. How many customer interviews to plan covers this in more detail.
Recruit the person who made the decision
Talk to people who lived through what you're studying. In a restaurant group that might be the controller, the finance director or an owner who still runs payroll; titles vary. Confirm early in each call what they personally did. Research screener questions helps you check fit before booking, and research participant recruitment covers channels and incentives.
Write the guide
GOV.UK's guidance on in-depth interviews suggests listing the topics you need to cover, ordering them so the conversation flows, writing a starter question and possible follow-ups for each, and testing the result on a colleague (GOV.UK). Pathfinder suggests no more than 15 main questions, open-ended rather than yes-or-no, with factual questions before opinion questions.
For the payroll study, a starter question might be: "Walk me through how you decided to change payroll providers. What happened first?" Follow-ups would trace who was involved, what they compared and what finally decided it. How to write a discussion guide walks through the whole document.
Run the interview
GOV.UK's advice for the session:
- Get informed consent and run through your introduction before you start.
- Begin with a few easy questions, such as what their job involves.
- Ask open, neutral questions, and follow up with simple prompts like "Can you tell me more about that?"
- Focus on stories and real examples rather than how things should happen.
- Stay quiet. The more you talk, the less they will.
- Don't follow the guide rigidly when something new and relevant comes up.
Leave time at the end to clarify anything you didn't understand. GOV.UK notes that interviews can take place in person or remotely by phone or video call. How to ask for recording consent covers recording.
Analyze and report what the interviews can support
Pathfinder lists the method's main limitations: answers can be biased, interviews take time to run, transcribe and analyze, and results usually can't be generalized because the samples are small and not random. Plan for each.
- Summarize right away. Pathfinder recommends summarizing the key information immediately after each interview, and checking facts you can verify independently. If a finance manager says their old provider charged per location, check that provider's pricing.
- Compare across people. Look for reasons that recur among the leavers and are absent among the stayers. Affinity mapping and how to analyze customer interviews cover this step.
- Report counts of people, not percentages. Pathfinder warns that numbers and percentages can give the impression that results apply to a whole population. "Four of the six leavers we interviewed switched after a payroll tax error" reports what you found. "67% of restaurant groups" claims more than six interviews can show.
Once the interviews show the main reasons, a survey or the company's own product data can tell you how common each one is. Qualitative market research and interviews versus surveys cover that sequence.
Your next step
Write down the decision your study needs to inform, the two or three groups of people whose experience bears on it, and five starter questions about things they did. Book three interviews per group, then decide whether you need more.
If the people you need are outside your customer list, as the payroll company's former customers are, Instant Expert can find people who match a description you write, such as "finance managers at multi-location restaurant groups." You review who it finds, it sends your invitations, and you pay only for calls that get booked. The finance professionals in restaurants and sales professionals in payroll software directory pages are one place to start.