In this post, we'll review a case study submission from one of our Exponent members, Abin Abraham.
Question: Estimate how much laundry detergent is used every year, across the world?
If you want to brush up on the basics of answering Estimation Questions, check out Exponent’s lesson on how to ace the estimation interview question.
Here is a framework that we recommend you use for Estimation questions.
- Clarify with Questions
- Know the Basic Numbers
- Decide on and Lay Down Your Structure
- Write Down the Equations
- Crunch Numbers
- Sanity Check
Abin’s Answers (with Commentary)
Clarify with Questions
- Is this for all types of detergents? Powder or liquid?
- Is this for detergents that are used in machines or ones that are used to clean by hand?
I am going to estimate detergents that are liquid and used by a machine.
Know the Basic Numbers
World population: 7.5B
Asia population: 4.5B
Africa population: 1B
Europe population: 1B
South America population: 500M
Oceania population: 40M
North America population: 500M
Number of people per household
South America: 4
North America: 3
Household size (calculated from above)
South America: 125M
North America: 170M
Total world household size: About 2B
Detergents and load size
Average bottle of liquid detergent = 100 fl oz
Usage per load = 4 fl oz
Loads per wash = 2
Editor’s Note: Abin’s submission didn’t explicitly lay down the answer structure and the equations he will be using, which we will cover in the Feedback section. We recommend you over-communicate and explicitly mention these to the interviewer upfront, for clearer communication and potential for the interview to help you along the way.
As an example, Abin could’ve communicated to the interviewer “I will first calculate the laundry load per bottle, then calculate how often people usually do their laundry (once a week), and use these numbers to determine the total number of detergent bottles a person will need for a year.”
Write Down the Equations
Editor’s Note: Same feedback as above. While I assume during an in-person interview Abin could use a whiteboard to lay down the equations first and then crunch the numbers, it is crucial for an interviewee to take it slowly and explicitly lay out the equations (rather than calculating as we go) to ensure the interviewer is following along every step of the way.
(Average volume of liquid detergent bottle) / (Usage per load * Load per wash) = (Sessions per bottle)
(Weeks in a year) * (Session per week) / (Sessions per bottle) = (Total bottles needed for a year per household)
(Bottles needed per year per household) * (Total households worldwide) * (Volume per bottle) = (Total volume of detergent used worldwide per year)
100 / (4)(2) = about 13 loads per bottle.
1 laundry session per week 13 loads = 13 weeks/bottle.
52 weeks in a year. Will need 4 bottles per year.
Bottles * fluid oz * households = 4 * 100 fl oz* 2B
Total Laundry detergent used worldwide per year = 800B fl oz
This is most likely an overestimate because I am assuming 100% of households have access to an electric washing machine. If 60% of the worldwide household population has access to an electric washing machine and used liquid detergent then the amount would fall to 480B fl oz
Feedback from Exponent’s PM Coach
What Abin did well
- Abin did a great job getting the basic numbers right, which became strong foundations for his answer throughout.
- Abin did a great job doing a sanity check at the end, correctly calling out his answer was an overestimation and why, and a reasonably adjusted answer.
What Abin could Improve On
- Abin could’ve articulated his structure and equations more clearly up front rather than laying down the basic numbers and going straight into calculation. Try not to leave the interviewer with any confusion about where you are going, and always over-communicate and check in.