What It Is
Assumption Mapping is a prioritization exercise that plots business and product assumptions on a two-by-two matrix. One axis represents how important the assumption is to the success of the idea (if this is wrong, does it kill us?). The other represents how much evidence currently supports it (do we know this is true, or are we guessing?). The technique is used in Teresa Torres's Continuous Discovery Habits and aligns with the validation thinking in Rob Fitzpatrick's The Mom Test.
When to Use It
- After generating a solution or product concept, when you need to decide what to validate first.
- When a team has a long list of unknowns and limited time to run experiments.
- During sprint planning for discovery work, to ensure the team is testing what matters most.
- When stakeholders push to "just build it" — the map makes hidden risks visible and concrete.
How It Works
Step 1: List Assumptions
For a given solution or initiative, brainstorm all the things that must be true for it to succeed. Cover four categories:
- Desirability — Do customers want this? Will they switch from their current solution?
- Viability — Can we sustain this as a business? Will it generate revenue? Is it legal?
- Feasibility — Can we build it? Do we have the technology and skills?
- Usability — Can customers figure out how to use it? Will they complete the key flows?
Step 2: Plot on the 2x2
Draw a matrix with two axes:
- Y-axis: Importance (bottom = nice to know, top = must be true or the idea fails)
- X-axis: Evidence (left = no evidence / pure guess, right = strong evidence / validated)
Place each assumption on the matrix based on team consensus.
Step 3: Prioritize the Top-Left Quadrant
Assumptions in the high importance, low evidence quadrant are your leap-of-faith assumptions. These are the ones that could kill your idea and that you currently know the least about. Test these first.
Step 4: Design Small Tests
For each priority assumption, design the cheapest, fastest experiment that could provide evidence. This might be a customer interview, a data pull, a smoke test, a prototype, or a concierge test.
Step 5: Update and Repeat
After running tests, move assumptions rightward on the matrix as you gain evidence. Re-prioritize and test the next most critical unknown.
Key Principles
- Test what scares you, not what is easy. Teams naturally gravitate toward assumptions they can test comfortably (feasibility questions for engineers, usability questions for designers). Force attention to the high-importance, low-evidence quadrant.
- Importance is about the idea, not the company. An assumption is "important" if being wrong about it means this specific solution will fail — not whether the whole company dies.
- Evidence is a spectrum, not binary. An assumption can move from "no evidence" to "some signal" to "strong evidence" over multiple rounds of testing.
- Do this as a team. Individual assumption maps reflect one person's blind spots. The exercise works best when product, design, and engineering plot together and debate placement.
Common Mistakes
- Testing only desirability assumptions. Teams often focus exclusively on "do customers want this?" while ignoring viability and feasibility risks that are equally fatal.
- Spending too long on the map itself. The map is a prioritization tool, not a deliverable. Spend 30 minutes mapping, then go run experiments. Do not wordsmith assumption statements for hours.
- Never updating the map. If you test an assumption but do not update its position, the map becomes stale and the team loses trust in it. Treat it as a living document.
Source
Teresa Torres, Continuous Discovery Habits (2021), Chapter 11 (Assumption Testing). Rob Fitzpatrick, The Mom Test (2013), Chapter 4 (discusses identifying what you need to learn before conversations). The 2x2 format appears across lean startup literature but is codified most clearly in Torres's work.
