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Assumptions in case interviews

Someone asked on Aug 10, 2018 - 3 answers


What is the best practice of making assumptions during an interview?

Stating the assumption and asking if it is a reasonable assumption to make to get a green light from the interviewer?

Thanks in advance!!

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Vlad updated his answer on Aug 10, 2018
McKinsey / Accenture / Got all BIG3 offers / More than 300 real MBB cases / Harvard Business School


I assume you are writing about market sizing questions. Here is the approach:

Let's say you need to calculate the number of dogs in Manhattan

  1. Make the assumptions based on the general knowledge or your personal experience (Use the example of your house where out of 100 apt-s 10 have dogs)
  2. Make the adjustment of the numbers to the situation in the particular case (NY is a busy city thus fewer households have dogs, so let's assume it's 5%)
  3. Check the assumption with the interviewer (Does it seem like a reasonable assumption?)



Benjamin replied on Aug 10, 2018
ex-Manager - Natural and challenging teacher - Taylor case solving, no framework


You have two challenges here.

1. You need to go fast

2. You need to be clear on the assumption your taking and the rational behind

In order to combine both, I recommand to take the lead on making assumptions quickly(no need to discuss it for hours), and explain why you're making it and if this is ok for the itw to move one with it.


Sidi replied on Aug 10, 2018
McKinsey Engagement Manager & BCG Consultant | Interviewer at McK & BCG for 7 years | Coached 60+ candidates secure MBB offers

Hi Anonymous,

the key is to always be crystal clear regarding your logic and the underlying driving assumptions! This is an absolute core skill to build for the case interview, since it directly mirrors how you are expected to work with clients. So you are right, sharing your assumptions and weaving it into how you discuss and align your approach is crucial! Failing to take along the interviewer (or later, the client) is one of the worst mistakes one can make. This is far worse than small math mistakes by the way!

Cheers, Sidi

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