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Case Interview

A practical example on how to 'crack' a case study interview

Mastering a case interview is not easy and requires a lot of self study and practice.

Below is a collection of some golden tips that are essential to keep in mind. There are tens of books and websites out there to help you prepare with the case. You can also explore some selected online resources and see options for interview coaching.

Below is also a worked example. If you want to make the most of it, try to solve each step yourself before reading the suggested solution. Good luck and have fun! :)

General tips

I summarize this with ASK - Ask, Structure, Kommunicate. Keep these in mind throughout your solution.

Ask

  1. Ask clarifying questions and listen

  2. Apply the 80/20 rule without too much detail

  3. Play back your understanding

Structure

  1. Build an issue tree for most questions or sub-questions

  2. Detail each branch based on 3 points

  3. Keep branches MECE: Mutually Exclusive, Completely Exhaustive 

Kommunicate

  1. Establish a dialogue, build a rapport and listen

  2. Voice out your thought process

  3. Try to have fun and communicate your passion for solving problems

Step-by-step QASSAR approach

I suggest you initially prepare separately each step. You can memorize it by remembering a Quasar (if you are an astrophysics enthusiast), just with an extra 's'...

  1. Question: what shall I exactly solve?

  2. Archetype: what basic structure could I adapt?

  3. Structure: what adapted structure will lead to the solution?

  4. Solution: how do I prioritize my deep dive?

  5. Answer: what is the answer to the initial question? 

  6. Risks: what could go wrong and is there any upside?

Stick to these for any case and you will be sure to be on the right track without missing important parts. Get feedback from others on which ones you still need to improve, then practice those ones even more. 

1. Question

You need to be sure you correctly understand the question and there is no misunderstanding before you proceed further. For example, imagine the interviewer was asking you: "How long does it take to go to Hawaii?". The most common mistake is to take this question at face value and start solving it straight away. You may or may not be answering the right question though. What about asking a few clarifying questions?


Try to solve this step by yourself, then see a possible approach below.

From where do I start?

  1. From this room

  2. From my house

  3. From the closest airport

Which means of transport do I use?

  1. A private rocket or airplane

  2. The quickest commercially available means

  3. Environmentally friendly means (e.g. Greta's sailing boat)

From when to when do I measure?

  1. From now until I reach my destination

  2. From when I start my travel until I reach my destination

  3. Just flight time plus transit time (no waiting time or door to door)

And you conclude by playing back to the interviewer: "The question is: What is the shortest time for me to go to Honolulu city center (Hawaii), leaving now from this room and using the quickest public means of transportation? Do I understand correctly?"

2. Archetype

These are the most common types of cases and my suggested way to approach them:

  1. Brainteaser, Consulting math: use an ad-hoc Issue Tree

  2. Market sizing, Profitability, post-merger synergies: use a Profitability Tree (or just its revenue or cost part)

  3. Market study/entry, M&A evaluation: use Porter's 5 Forces, Marketing 4Ps, 4Cs, or anything else you liked from your business strategy courses.

Note that some cases may contain a combination of the above. For example an M&A evaluation with calculation of  post-merger synergies.

While it is good to learn some general structure, a common mistake is to try to force-fit a general structure into a specific problem.

3. Structure

This is in my opinion the most important step and where I see the most candidates failing. Take the time to think, write the structure down and use it as a physical map in front of you to stay on track during the solution. Adapt the basic structure from the archetype to the actual case at hand. This will be the map you will follow.

Try to solve this step by yourself, then see a possible approach below.

In this case you need to build an ad-hoc solution tree. The total time is the sum of the following:

  1. From origin to flight departure

    1. Preparing to leave (e.g., what is my citizenship? what do I need to bring? do I need to pick up my passport at home?)

    2. Reaching the airport (e.g., train or taxi? Which airport?)

    3. Checking in and waiting time at the airport (e.g. what is shortest flight? can I catch it today?)

  2. Flight 

    1. Scheduled Flight(s)'s time​: total flying time

    2. Scheduled Transit(s)' time: waiting time between connecting flights

    3. Delays, which may affect any of the two above (e.g., expected flight disruptions from strikes, frequent delays on the route)

  3. From flight landing to destination

    1. Immigration, depending on the traveler citizenship and Visa pre-work (e.g., did a non-US citizen procure his/her ESTA in advance?)

    2. Luggage collection (if applicable)

    3. Travel to city center (e.g. what is the best way to go from Honolulu airport to the city center?)​

4. Solution

With the map (structure) in front of you, now it is time to venture into the territory (the actual solution)

You may get lost in details, so keep track of time and ask the interviewer if there is a specific aspect of the case where he wants you to do a more detailed calculation.

Try to solve this step by yourself, then see a possible approach below.

In this case, you may start by prioritizing point #2 of the issue tree, i.e. the flight time calculation, because:

  1. The flight time will certainly represent the majority of the travel time

  2. The choice of flight may determine some some of inputs to solve the other parts (e.g., from which airport in New York do I need to leave?)

  3. If we run out of time, we could give a better 80/20 answer 

5. Answer

Make sure you actually answer the initial question. Read it again if necessary and paraphrase it in the answer, so you make sure you don't miss the actual point.

Try to solve this step by yourself, then see a possible approach below.

Your final answer: "The shortest time to go to Honolulu city center leaving now from this room using the quickest public means of transportation is approximately 50 hours."

Here you may realize that you forgot to include point #3 of the initial issue tree, i.e. the 'last mile' time from landing in Honolulu airport to getting to the city center. Paraphrasing the initial question may seem pedantic but can save you from such mistakes.

6. Risks

This is the cherry on the cake: you would get some bonus points if you don't just stop at the final answer, but add a few considerations of what could go wrong (risks) and what could be the upside (opportunities). Show that you can think both logically and creatively.

Try to solve this step by yourself, then see a possible approach below.

For example, the 50 hours calculation is heavily influenced by the fact the shortest flight only leaves tomorrow evening, and we had to take a sub-optimal flight leaving this morning. On the other hand, the connection in Los Angeles is only 90 minutes.

Are we really optimizing the travel time? We may ask for example:

  1. What if we miss the connection? e.g., if the first flight is delayed, when would be the next connecting flight?

  2. Could we optimize the departure time? e.g., what is the reason to leave immediately vs. wait for a better/shortest/cheaper flight?

  3. Are there some parameters that are more important? e.g., could we spend extra 5 hours in the Los Angeles transit, if the flight cost 30% less

  4. Could we optimize the experience while getting there or returning? e.g., maybe on the way there you can meet other people, or take advantage of a long transit time to visit your relatives in Los Angeles

Closing the case on a positive and creative note leaves a good last impression to the interviewer.