You're asked to size a large, ambiguous initiative with no detailed spec yet, to decide whether it even fits this year. How do you produce a credible estimate?
role-specific · Senior level · product-management
What the interviewer is really asking
Assesses whether a senior PM can produce a defensible order-of-magnitude estimate for an under-specified initiative through decomposition, reference-class comparison, and explicit assumptions rather than refusing or guessing.
What to say
- Decompose the initiative into its major chunks of work and size each roughly with engineering, because a sum of coarse component estimates is far more defensible than one gut number for the whole.
- Anchor against reference classes — how long did similar-shaped initiatives actually take here — to counter the inside-view optimism that makes from-scratch estimates run low.
- Express the result as a t-shirt size or order of magnitude with the assumptions and the biggest unknowns stated, plus a cheap next step (a short discovery phase) that would convert the rough number into a planning-grade one.
What to avoid
- Don't refuse to estimate because the spec isn't ready; leadership needs an order-of-magnitude read to make a portfolio call, and 'I can't estimate that' isn't an answer at senior level.
- Don't produce a falsely precise number — '14.5 weeks' off a one-paragraph brief signals you don't understand how much uncertainty you're carrying.
- Don't estimate it alone from the inside view; skipping engineering input and comparable past projects is how teams systematically underestimate big bets.
Example answers
Strong: Asked whether a multi-region expansion fit this year, I broke it into the obvious pieces — data residency, latency infra, localized billing, compliance — and got rough engineering sizing on each rather than one number for 'expansion'. I cross-checked against our last major infra project, which had run 40% over its first guess, and used that to inflate my estimate honestly. I came back with 'large, roughly two quarters, with compliance as the biggest unknown' plus a proposed three-week discovery to firm it up — enough for leadership to make the portfolio call without pretending we had certainty.
Weak: Without a real spec I can't give a meaningful estimate, so I'd tell leadership we need the requirements fully defined before I can size it at all.