What is the interviewer looking for?
In estimation interview questions, interviewers are evaluating your problem-solving and quantitative skills. They are looking for a ballpark number, not an accurate number. What matters is whether you are logical, can explain all your assumptions clearly, are organized in your work, and good with numbers.
How to structure your answer?
The book Cracking the PM Interview by Gayle Laakmann McDowell and Jackie Bavaro provides a good suggestion on how to structure an answer for this type of question. It can be summarized as:
- Ask clarifying questions. This will eliminate any ambiguity of what should be included in your calculations.
- Make an equation. Consider edge cases or alternate sources of data and write down any facts that you know will help with calculations.
- Break down the equation into components. Write down any assumptions next to the components so you don’t forget to explain them clearly to the interviewer.
- Do the math. Calculate the result of each component and compute the result.
- Do a sanity check. Do your results make sense? If not, recheck your equation, assumptions and arithmetic.