Great news – you got an interview. Congratulations!

Although TRSM has prepared you and given you the foundation to be successful you must still get ready for the interview. A very specific type of interview that requires you to do some planning is the case interview. There was a time when only consulting firms used this method but larger firms are seeing the importance of assessing your problem solving skills before they hire you.

There are generally two aspects of a case interview: a guesstimate or marketing sizing and a short business case.

For this blog post, let’s tackle guesstimates!

This tests your ability to work quickly under pressure, to manipulate numbers, to be logical and to be able to have a conversation with the interviewer while you are working on your calculations. Be sure to bring some paper or cards to the interview to record the key information or write out your calculations.


Here is an example of a question: How many cups of coffee were consumed in Canada in the last week?

As you can see, the question may or may not relate to the industry in which the firm operates or even have anything to do with the job for which you are interviewing.

Step 1: Paraphrase or summarize the question out loud to the interviewer.

Example: “You are asking me to estimate how many cups of coffee were consumed in Canada in the last week?” Note the words that are in bold. Each of these represents a place where by misunderstanding the question you could provide the wrong answer.

You may or may not be allowed to ask additional questions of the interviewer. So, you should ask an important first question, “May I ask you some questions?”

Step 2: Confirm what the question is asking you for

By asking questions you are reflecting your understanding of the complexity of business situations. For example, confirm that the question is asking about the number of cups per day across all days of the week, that is 7 days a week. You could also ask if this includes all types of coffee including the ones with alcohol. Does it include such hot coffee or also ice cappuccinos?

Step 3: State your assumptions/ Make assumptions

State your assumptions aloud to the interviewer. You may assume that the same number of cups are drunk each day or that fewer cups are drunk on weekends or holidays, as people are not working and might drink less coffee.

Also, make some assumptions about the percent of the Canadian population that drinks coffee. Start by giving an approximate number for the population, 30 million. Using a round number makes the calculations easier.

For example, I would assume 20% of the population are children under 16 who do not drink coffee and that 20% of the population don’t drink coffee at all.

So then you would say: I am going to assume that 60% of the population drinks coffee. The calculation would be 0.6 x 30,000,000, x 1 cup a day x 7 days = 126,000,000. [Aren’t you glad you used round numbers?]

You could stop with that answer however, a more sophisticated answer would include a calculation that demonstrated your ability to use averages in the calculation.

Step 4: Go that extra mile

You might say, of the 60%, I am going to assume that 50% of coffee drinkers consumer 2 cups a day, 25% consumer 4 cups a day and 25% consumer 1 cup a day.

The calculation would look like this: 0.5 drink 2 cups per day, 0.25 drink 4 cups per day, and 0.25 drink 1 cup per day. The average is: (2×0.5) + (4×0.25) + (1×0.25) = 2.25 cups/coffee drinker/day.

Then you would calculate the number of cups per day of coffee:

0.6 × 2.25 × 300,000,000 = 405 million cups

And finally the number of cups per week: 405 million cups × 7 = 2.84 billion cups (~ 2.8 billion cups)

You can see the difference between the two answers when you increase the number of cups of coffee drunk per day.

See it wasn’t so bad was it? It may seem a little silly at first, but I can ensure you that many companies are using guesstimates when assessing a potential employee.

Be sure to keep an eye out for my second post on short business cases! It will be a must read for anyone looking to be success in a job interview and landing the job you want!

Share your interview experiences with me in the comments below: the good, the bad, and the ugly!

Posted by Joanne McNeish

Prior to joining TRSM, Dr. McNeish held senior research, marketing and advertising positions in public and private sector companies. In 2002, she was made a MRIA Fellow which recognizes those who have made distinguished contribution to marketing research in Canada. Joanne was one of the youngest candidates and one of only a few women to be so honoured. She received her PhD from Carleton University in 2010.

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