✏️ Explanatory Question

The interviewer says your estimate is off by 10x. How do you respond?

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Answer with Explanation

Stay calm - this is usually a deliberate test of whether you defend numbers blindly or reason about them.

1. Do not immediately capitulate or dig in. Both are wrong. Say: "Let me re-check the assumption that drives it."

2. Re-derive out loud, isolating each input. A 10x error almost always comes from one of a small set of causes:

  • Bits versus bytes - an 8x error, which looks like 10x when rounded.
  • A dropped or extra factor of 1,000 - MB versus GB.
  • Seconds per day - using 100,000 instead of 86,400 is only 16%, but using 3,600 gives exactly 24x.
  • Per-user versus total - forgetting to multiply by user count.
  • Missing the peak multiplier, or missing replication.
  • A genuinely different assumption - they may think users send 100 messages a day, not 10.

3. Separate arithmetic errors from assumption disagreements. If the arithmetic is wrong, fix it and thank them. If they simply assume different inputs, say so explicitly: "My maths holds for 10 actions per user. If it is 100, everything scales up 10x - and that changes the design, because at that point one cache node is no longer enough."

4. Show what the correction changes. The most valuable response is: "With the corrected number, my earlier conclusion that we could fit this in one database no longer holds - I would need to shard." That demonstrates the estimate was actually load-bearing rather than decorative.

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