Every case interview guide tells you to "state your assumptions." It’s table stakes. Yet, most candidates treat it as a box-checking exercise, rattling off numbers without context. This is a critical error. Your assumptions aren’t a preamble to the analysis; they are the analysis. Good candidates list them. Elite candidates construct a logical, defensible "Assumption Stack."

The Problem with "Flat" Assumptions

A typical candidate doing a market sizing problem sounds like this: "Okay, I'll assume the US population is 330 million. I'll assume 75% are adults. I'll assume 50% of them will buy this product. And I'll assume they buy it twice a year."

This is a flat structure. The numbers are disconnected statements of fact. There is no underlying logic exposed, forcing the interviewer to guess at your reasoning. If one number is wrong, the interviewer simply corrects you. There's no discussion, just a pass/fail on your knowledge. This approach completely misses the point of the exercise, which is to showcase your structured thinking, not your memory of census data.

The Elite Alternative: The Assumption Stack

An Assumption Stack is not a list; it’s a logical construct. Each assumption is a building block that rests on the one below it, and crucially, each block comes with a brief, explicit justification. It transforms a series of guesses into a transparent, logical model that the interviewer can engage with.

An effective Assumption Stack has two core components:

• Logical Tying: Each assumption flows directly from the previous one, creating a clear, top-down path from the macro number to the final answer.
• Explicit Justification: For every number you use, you provide a concise "why." This rationale is more important than the number itself.

Building Your Stack: A Market Sizing Example

Let's use a classic prompt: "Estimate the annual market size ($) for replacement electric toothbrush heads in the United States."

The Standard (Weak) Approach:

"I'll assume a population of 330M, 80% use electric toothbrushes, and they replace the head 4 times a year at $10 a head." This is a black box. The logic is hidden.

The Assumption Stack (Elite) Approach:

"I'll build my estimate from the top down, starting with the total population and segmenting from there. My core assumptions will be about user demographics, adoption rates, and replacement frequency."

• US Population: "Let's start with the US population, which is roughly 330 million. I'm using this as a stable, commonly accepted macro number."
• Target Population: "From there, I'll exclude children under 10 and the very elderly, who are less likely to be primary users of electric toothbrushes. Let's estimate this removes about 20%, leaving a target population of ~260 million."
• Addressable Population (Income): "Electric toothbrushes are a premium product. I'll assume they are primarily used by middle-to-high income households. I'll estimate this is about 50% of the target population, which gives us an addressable market of 130 million people."
• Penetration Rate: "Within that addressable group, not everyone will own one. It's a mature but not fully saturated market. I'll assume a penetration rate of 60%, giving us ~78 million users."
• Replacement Frequency: "Dentists recommend replacement every 3 months, so 4 times per year. User behavior likely lags, so I'll assume an average of 3 replacement heads per user per year."
• Price per Unit: "These heads are often sold in packs, but the per-unit price is likely around $10. I'll use this as a baseline."

This structure allows the interviewer to see your logic, not just your numbers. They can now engage at a higher level: "That's a reasonable income segmentation. Have you considered how that might differ between urban and rural populations?" You are now having a collaborative, partner-level discussion.

Why The Assumption Stack Wins

• It Showcases Your Logic: It proves you have a structured, reasoned approach to ambiguity, which is the core skill of a consultant.
• It Invites Collaboration: Justifying your "why" gives the interviewer hooks to engage with. It turns a potential interrogation into a joint problem-solving session.
• It’s Defensible and Modular: If the interviewer challenges your penetration rate, you can adjust that single block without demolishing your entire framework. You can say, "Great point. If we adjust penetration to 70%, that increases our user base by X, which has Y effect on the final market size."
• It Builds Credibility: A well-constructed stack signals to the interviewer that you are rigorous, thoughtful, and capable of building a client-ready model under pressure.

The Bottom Line

Stop listing assumptions and start building your Assumption Stack. This technique elevates your quantitative analysis from a simple calculation to a demonstration of structured, defensible, and collaborative problem-solving. It’s a clear signal to your interviewer that you don’t just find answers—you build the logic that leads to them.