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Below we outline twelve (12) tips for nailing the guesstimate question:
1. Practice
Practice doing some guesstimate questions before the interview so that you are prepared. It is a good idea to bring a pen and graph paper to the interview so that you can keep track of your calculations.
2. Know some basic facts
Below are some facts that you should know. In addition, you should also know the key demographics for the country where you are interviewing.
3. Use round numbers
You are responsible for doing the calculations so pick numbers that are easy to work with. For example, estimate the population of America as 300 million not 316.1 million.
4. Clarify units of measurement
You should clarify the units of measurement that the interviewer wants from you. For example, market size can be measured by revenue or sales volume.
5. Take a moment
It is important to maintain your composure so before starting to answer the guesstimate question take a moment to write down the key details of the question, and consider your approach.
6. Have a clear approach
It is important to have a clear approach to help you answer the question. More on this in a later post.
7. Ask questions
Your interviewer may be able to provide you with direction. If the question is “How many ping-pong balls will fit inside a 747?” the first question you might ask is “What is the volume of a ping-pong ball?”
8. State your assumptions
The interviewer may not know the answer or may not want to give you direction so you’ll have to make assumptions.
It is a good idea to clearly state your assumptions. For example, “let’s assume that the diameter of a ping-pong ball is 4cm. The formula for the volume of a sphere is 4/3.pi-r^3. So the volume of a ping-pong ball would be about 11-pi centimetres cubed”.
9. Think out loud
The interviewer is trying to assess your thought process in getting to the answer, not the answer itself. If you don’t think out loud, you make it difficult for the interviewer to give you points.
10. Explain your logic
As you make your way through the problem it is helpful to explain the logic behind each of your assumptions.
Instead of saying “a 747 is about 100 metres long” you could say “I know that an average car is about 5 metres long and based on my experience I would say that 20 cars lined up end to end would be about the same length as a 747. So I will assume that a 747 is 100 metres long”.
11. Answer the question
After doing all of the calculations remember to answer the question that has been asked.
12. Be prepared for follow-up questions
After you answer the guesstimate question, the interviewer might ask “If you had to find the real answer to the question, how would you do it?” This is a test of your creativity and resourcefulness.
[For more information on consulting interviews, please download “The HUB’s Guide to Consulting Interviews“.]