Check units and scales for consistency
Catch order-of-magnitude errors by ensuring your units are consistent across the estimate.
Key takeaways
- What it is: Catch order-of-magnitude errors by ensuring your units are consistent across the estimate.
- Why it works: Most large estimation errors are unit or scale errors: confusing thousands with millions, minutes with hours, or individuals with households. Units act as an error-detection system — when the units of the product do not match the units of the answer, something is wrong. Tracking units explicitly throughout the decomposition catches these errors before they produce a wildly wrong answer.
- Evidence: Plausible mechanism, limited direct outcome data.
- Avoid: Carrying units in your head rather than writing them down — unit errors are almost universally caught by writing them and almost universally missed when they are implicit.
Why it works
Most large estimation errors are unit or scale errors: confusing thousands with millions, minutes with hours, or individuals with households. Units act as an error-detection system — when the units of the product do not match the units of the answer, something is wrong. Tracking units explicitly throughout the decomposition catches these errors before they produce a wildly wrong answer.
How to do it
- 1Label every sub-estimate with its units (people/household, minutes/day, dollars/unit).
- 2Verify that the units of each product combine to give the units of the quantity you want.
- 3If units do not work out, find the step where they went wrong.
- 4As a sanity check, compute whether the order of magnitude seems consistent with an everyday anchor.
What the evidence says
MechanisticDimensional analysis is a foundational technique in physics and engineering for catching errors in calculations. Its application to estimation is mechanistically straightforward and widely taught in quantitative reasoning curricula.
Honest caveat: Unit-checking catches scale errors but not errors in the values themselves; correct units with wrong numbers still produces a wrong answer.
Common mistake
Carrying units in your head rather than writing them down — unit errors are almost universally caught by writing them and almost universally missed when they are implicit.
IX Coach traces the units in your estimation chain and flags when sub-estimates combine inconsistently.
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