How Expert Interviews Make Market Sizing Defensible

Every market sizing exercise eventually hits the same wall. The analyst has pulled the industry reports, combed through public filings, and cross-referenced three different data providers. The estimates are in hand. And they disagree by a factor of two.

That is not a data problem. It is a definition problem, a scope problem, and an assumptions problem. Secondary research cannot resolve it, because the reports that produced those conflicting numbers used different boundary conditions, different revenue capture definitions, and different views on what counts as addressable demand. No amount of additional desk research will reconcile them.

This is precisely where expert interviews enter the market sizing process. Not as a way to generate the number, but as the only reliable mechanism for resolving the disagreements that secondary sources cannot settle on their own.

For private equity investors, corporate strategy teams, and consulting firms building market models that need to survive investment committee scrutiny, the distinction matters. A market size derived from secondary research alone is a reported figure. A market size validated and calibrated through expert interviews is a defensible one.

This guide explains the specific roles expert interviews play in market sizing: what they resolve, when to deploy them, which expert profiles answer which questions, and how to integrate their input into a model that holds up under pressure.

What Secondary Research Cannot Tell You

Secondary research is a starting point, not a finishing point. Industry reports, analyst databases, and public filings answer a narrow set of questions well: historical revenue figures for named companies, broad category growth rates, and publicly disclosed market share data. They answer poorly, or not at all, the questions that actually determine whether a market model is accurate.

The three categories of information that secondary research consistently fails to resolve are:

• Market boundary definitions. Does the TAM include adjacent services that some competitors bundle but others charge separately for? Does it capture channel revenue or only direct sales? Two reports can describe the “same” market and arrive at radically different numbers because they drew the boundary differently. An expert who has operated inside the value chain knows where the real boundary sits.
• Unit economics and pricing dynamics. Published figures often reflect list prices or blended averages that obscure actual transaction economics. Experts reveal the discount structures, volume thresholds, and contract mechanics that determine what buyers actually pay and what sellers actually retain.
• Demand-side adoption rates. Reports project growth based on historical trends. They cannot account for the procurement inertia, switching costs, or budget cycle constraints that an experienced operator knows are slowing or accelerating adoption in a specific segment.
• The core problem: Secondary research tells you what has been reported. Expert interviews tell you what is actually happening. For a market sizing exercise to produce a defensible number, it needs both.

These gaps are not marginal. In contested or niche markets, the difference between a secondary-only estimate and an expert-calibrated one can exceed 30 to 50 percent, enough to change an investment thesis entirely. For a deeper look at when to bring in outside research support, see 5 signs you need to hire a market sizing research agency.

The Five Specific Roles Expert Interviews Play in Market Sizing

Expert interviews are not a single tool applied uniformly. They serve distinct functions at different stages of a market sizing engagement. Understanding which role is needed at which point determines which expert profiles to recruit and what questions to ask.

1. Resolving Conflicting Estimates

When two credible secondary sources produce materially different market size figures, the answer is almost never in a third report. The answer is in understanding why the definitions diverge. An expert who has spent years operating in the space can explain, in a single call, whether one estimate is capturing distribution channel revenue that the other excludes, or whether one report’s scope includes a regulatory category that the other treats as a separate market.

This is the most time-critical use of expert interviews. In a deal window, a team cannot afford to spend weeks triangulating reports. A well-scoped 45-minute call with the right operator can resolve the discrepancy in the time it takes to read another analyst brief. The speed advantage is real: 72-hour expert turnarounds have become a measurable M&A advantage precisely because they compress the most time-sensitive part of the diligence process.

2. Calibrating Bottom-Up Assumptions

Bottom-up market sizing builds from unit-level economics: average selling price, purchase frequency, customer count by segment, and channel take rates. Each of those inputs is an assumption. Secondary research provides rough proxies. Expert interviews provide calibrated figures drawn from direct operating experience.

A former distribution executive, for example, can confirm or correct the channel margin assumptions that underpin a bottom-up model. A procurement director at a major buyer can validate the average contract size and renewal cycle that the model treats as a given. These are not marginal refinements; they are the inputs the model is most sensitive to.

3. Identifying Structural Demand Constraints

Growth projections in secondary reports are typically extrapolations of historical trends. They do not reflect the structural constraints that practitioners know are limiting or accelerating demand in real time. Expert interviews surface these constraints directly.

Common examples include:

• Regulatory approval timelines that cap adoption in healthcare or financial services markets
• Workforce or infrastructure bottlenecks that limit how fast a technology can actually be deployed
• Procurement cycle rigidities that mean published growth rates will not materialize for two to three years

Customer concentration dynamics where three buyers represent 60 percent of the market, making the TAM less accessible than the headline figure suggests

4. Pressure-Testing the Addressable Market Boundary

TAM figures are only useful if the addressable market is correctly scoped. Expert interviews are the primary mechanism for pressure-testing whether the SAM (serviceable addressable market) assumptions in a model are realistic.

A channel partner, for instance, can confirm which customer segments are actually reachable given current distribution infrastructure. A technical operator can explain which use cases in the broader TAM require capabilities the market has not yet developed, effectively shrinking the near-term addressable opportunity. These distinctions are rarely visible in secondary data.

5. Validating Growth Thesis Assumptions

Every market sizing exercise that is used for investment or strategy purposes rests on a growth thesis: the market will grow at X percent because of Y driver. Expert interviews test whether practitioners actually believe that thesis, and whether the assumed drivers are operating as expected.

In one documented case from a sell-side diligence engagement, a research team built a survey-driven market model using 30 expert interviews and a 200-respondent survey, ultimately sizing a market at approximately $2.75 billion growing at roughly 5 percent CAGR. The expert interviews were not the source of those numbers; they were the validation layer that confirmed the growth drivers were real and the assumptions were grounded in how the market actually operated. The analysis led the client to defer a planned sale and pursue an emerging-market growth strategy instead, a decision that would not have been credible without the expert-calibrated model behind it.

Which Expert Profiles Answer Which Market Sizing Questions

The quality of a market sizing exercise is only as good as the quality of the experts recruited for it. The right profile depends entirely on which question the interview is designed to answer. Recruiting generically, or defaulting to the most prominent names in a sector, produces calls that are interesting but not useful.

The table below maps the most common market sizing questions to the expert profiles best positioned to answer them.

The Recency Requirement

Expert profile matching is not just about role. It is about recency. A former VP of Sales who left a market three years ago can describe the market as it was; they cannot reliably describe the dynamics that are operating today. For market sizing purposes, the most valuable experts are those with current or very recent direct exposure to the specific segment being modeled.

This is where standard expert network searches often fall short. Keyword-matched profiles may surface credentialed individuals whose experience predates the market conditions the model needs to reflect. Custom recruiting for PE due diligence, which starts from the specific question and works backward to the profile, consistently produces more relevant results.

Panel Composition Matters as Much as Individual Quality

A market sizing panel composed entirely of supply-side operators will produce a systematically optimistic view of addressable demand. A panel composed entirely of buyers will underestimate the market’s capacity to reach them. The most reliable panels deliberately mix perspectives:

• Supply-side: Former executives at market participants who understand revenue recognition and competitive dynamics
• Demand-side: Procurement leaders or end users who understand actual buying behavior and budget allocation
• Channel: Distributors or intermediaries who see both sides of the transaction and can reconcile the two

Mixing these perspectives does not average out to a mediocre view. It produces a triangulated view that is more defensible precisely because it was stress-tested from multiple vantage points.

How to Integrate Expert Interviews Into a Market Sizing Model

The interview itself is only part of the work. How the insights from expert conversations get translated into model inputs determines whether the exercise produces a defensible output or simply a collection of interesting perspectives.

Sequence: Interviews Before the Model, Not After

A common mistake is treating expert interviews as a validation step at the end of the sizing process, after the model has already been built. This produces confirmation bias. The team has already committed to a structure and a set of assumptions, and the interviews are unconsciously filtered through that frame.

The more rigorous approach runs interviews before the model is finalized, using the conversations to set the boundary conditions and calibrate the key inputs before they are locked in. The sequence that consistently produces the most defensible outputs is:

• Secondary research sweep to establish the range of existing estimates and identify the specific points of disagreement
• Expert interviews to resolve boundary definitions, calibrate unit economics, and surface structural constraints
• Model construction using expert-calibrated inputs rather than secondary-derived proxies
• Validation round with a smaller set of experts to pressure-test the model’s conclusions before finalization

Synthesizing by Question, Not by Call

The most common failure in expert interview synthesis is organizing findings by expert rather than by question. The result is a series of individual perspectives that are difficult to weigh against each other and impossible to translate directly into model inputs.

Effective synthesis lines up every expert’s response against each key model assumption. Where experts converge, that convergence is strong signal. Where they diverge, the divergence is itself informative: it usually indicates a genuine market dynamic (such as regional variation or segment-specific behavior) that the model needs to reflect rather than average away.

A practical rule: weight expert views by proximity to the evidence. A current regional sales director’s view on adoption rates in their geography carries more weight than a former executive’s view of the same dynamic from two years ago. Recency and directness of exposure are the two most important weighting factors.

When to Supplement with Quantitative Primary Research

Expert interviews are depth instruments. They produce rich, nuanced views on judgment-heavy questions. They do not produce statistically representative data on how widely a view holds across a population.

For market sizing exercises where the model is sensitive to adoption rates or purchase behavior across a large, heterogeneous buyer population, expert interviews should be paired with a structured B2B survey. The interviews shape the hypotheses and calibrate the key inputs; the survey tests how broadly those inputs generalize. Together, they give an investment committee both the narrative and the numbers, which is the standard that serious diligence programs are now expected to meet.

The IC-Readiness Standard

The practical test for any market sizing exercise used in investment or strategy contexts is whether it can survive informed scrutiny. Investment committees, boards, and senior strategy leaders will probe the assumptions behind the number, not just the number itself. Secondary-only estimates fail this test because they cannot explain why the boundary was drawn where it was, or why the unit economics assumptions are correct rather than convenient.

Expert-calibrated models can answer those questions, because the boundary definitions and unit economics came from practitioners who operate at those boundaries. When an IC asks “why did you size the market this way,” the answer “because three independent operators with current exposure to this segment confirmed these boundary conditions and calibrated these inputs” is a different class of answer than “because the analyst reports said so.”

The defensibility gap is real. Growth equity funds have passed on IC approval for market size figures that could not be explained from first principles. The same figures, rebuilt with primary expert input, have passed on first presentation. The number did not change; the methodology behind it did.

For teams operating in niche, fragmented, or rapidly evolving markets where secondary data is sparse or unreliable, expert interviews are not a supplement to the market sizing process. They are the process. The secondary research provides the starting frame; the expert conversations provide the substance that makes the frame defensible.

The question is not whether to use expert interviews in market sizing. The question is whether the experts recruited are current enough, specific enough, and diverse enough in their vantage points to produce a model that reflects how the market actually works.

That starts with recruiting the right experts for the right questions, which is a more precise exercise than most teams treat it as. Nexus Expert Research specializes in custom-recruiting high-level subject matter experts for exactly these engagements, sourcing niche operators from scratch for every project rather than drawing from a static database.