How to Estimate With Missing Information in an Inter­view

Some interview prompts ask for a number when the facts are missing, for example "How many printed programmes would you prepare for a talk?" Without enough inputs, more than one estimate may be reasonable. What you can show is a method: separate what you are told from what you assume, show the arithmetic, give a range and update it. This guide is not a purchasing guide, and it cannot promise that any answer will be judged correct.

The practice suggestions here are optional. Honesty, confidentiality and permission are not optional. The worked example uses invented figures. They are not research and not a claim about real events.

If you are unsure what a question means, see our guide to asking for clarification. For a work scenario, not an estimate, see our guide to situational interview questions.

A number alone is not an answer

A bare number cannot be checked. Give it with its unit, the inputs behind it and the input you are least sure of. While information is missing, a tentative range is more honest than a single figure. If you are asked to commit, say which figure you would commit to and what would change it. Do not assume that a standard allowance applies to the prompt. If you add a margin, name it as your own assumption and say why.

Get the unit right

Estimates can go wrong when the unit changes halfway through. Registrations are people who signed up, attendance is people who turn up, and programmes are the items printed, which may serve one person or several. Say which you are estimating, and convert between them deliberately.

Separate facts from assumptions

Keep two lists: what you were told, and what you are assuming. Do not move an assumption into the first list because it sounds likely, and do not call it typical unless you know that.

Find the input that matters most, then update

Change one input at a time and see which moves the answer most, and ask the interviewer about that one first. If they cannot say, carry on with a labelled assumption. When a new fact arrives, say what changed and redo the arithmetic aloud. Do not defend your first figure.

A fictional worked example

Every figure below is invented to show the method. Do not present any of it as your own experience or copy it.

Fictional prompt: "A community centre is holding a talk next month and asks how many printed programmes to prepare. How would you work it out?"

Fictional answer, part one: "First, the unit. I will assume one programme per person who attends, not per person who registers. Please correct me if programmes are shared. I have been told nothing so far, so I would like to know how many people have registered, whether the hall has a limit and whether more could be printed later."

Fictional interviewer reply: "Sixty people have registered so far. Registration is still open. I cannot tell you about printing more later."

Fictional answer, part two: "The supplied fact is 60 registrations. Everything else is my assumption. I assume between none and 15 of the registered people do not come, and between none and 10 people come without registering. I have no data for either, so they are placeholders."

• Lower case: 60 registered, 15 do not come, nobody walks in. 60 − 15 = 45 people, so 45 programmes.
• Upper case: 60 registered, all come, 10 walk in. 60 + 10 = 70 people, so 70 programmes.

Fictional answer, part two, continued: "My tentative range is 45 to 70. The input that matters most is one I cannot see: registration is still open, so 60 may grow."

Fictional new fact from the interviewer: "Registration has reached 75. The hall admits no more than 80 people."

Fictional answer, part three: "Now the supplied facts are 75 registrations and a limit of 80 people. With the same assumptions the lower case is 75 − 15 = 60. The upper case would be 75 + 10 = 85, but the hall admits 80, so the upper case is 80. The range is now 60 to 80, and 80 is a ceiling set by a supplied fact, not by my guess. If two people shared each programme, the same cases would need 30 and 40. If I had to name one figure today, I would name 80 under my one-programme-per-attendee assumption. Whether I would aim lower or nearer that ceiling depends on something I do not know: whether more could be printed later, and at what cost. I would ask that next."

Practise your answer

Make up estimates with invented figures. Say the unit, the supplied facts, the assumptions, the arithmetic and the range aloud. If you practise with another person, agree with them before recording and before sharing any recording. In a real interview or assessment, follow any rule about calculators, and do not use any tool to generate answers unless the organiser has explicitly allowed it.

This guide is published by Muqabala. You can visit our interview practice page for your next practice session. Do not paste confidential employer, client or personal information into any practice tool.