What is the size of the prescription glasses market in France?
Nobody expects the true figure. The interviewer is assessing the logic of your estimate: a defined scope, a clear breakdown, assumptions stated and justified, a clean calculation and a final check. An estimate off by a factor of two but built methodically is a good answer.
What the interviewer is assessing
- That you define what you are counting. Pairs sold or revenue, per year, lenses and frames or frames only: the question is often ambiguous on purpose.
- That you announce your breakdown before putting numbers in. The interviewer should see the whole calculation tree before the first figure.
- That your assumptions are reasoned. "I assume 60%" is not enough: it needs a reason, however brief, drawn from what you observe around you.
- That you round to compute quickly, and set the result against a reference point to see whether it stands up.
How to build your answer
- 1
Pin down the scope
Ask one or two questions, or state your choice: "I will estimate annual revenue from prescription glasses, lenses and frames included." - 2
Choose your approach and announce it
From demand, you start with the population and work down to buyers. From supply, you start with the number of outlets and their activity. Take the one where your assumptions are strongest. - 3
Set the assumptions and calculate
One assumption per level, justified in a few words, with round numbers. Segment when it really changes the result, for example by age group. - 4
Check and comment
Bring the result down to spend per inhabitant or revenue per shop, and say which assumption weighs most.
How a good answer starts
"I will estimate annual revenue from prescription glasses in France, lenses and frames included, starting from demand. My calculation: the population, times the share of people who wear prescription glasses, divided by the time between two purchases, times the average price of a pair.
I start from about 68 million inhabitants. For the share of wearers I reason by age: almost everyone wears them after fifty, a minority among children, roughly half in between. I take 60% on average, so about 40 million wearers. I assume people change glasses every three years, which gives roughly 13 million pairs a year. With an assumed average price of 300 euros a pair, I get to about 4 billion euros. As a check, that is close to 60 euros per inhabitant per year, which seems plausible to me for an everyday health product. The most sensitive assumption is the average price, which varies a lot with the lenses."
This example is made up: it shows the tone and the construction, not a script to memorise. Your answer has to come from your own experience or from the data the interviewer gives you.
Common mistakes
- Starting to calculate without having defined the scope or announced the method.
- Giving assumptions without justifying them, or asking the interviewer to supply every figure.
- Keeping awkward numbers that slow the calculation and multiply errors, when rounding does not change the order of magnitude.
- Announcing the result without checking it or saying which assumption moves it most.
Follow-ups to expect
"How would you check that figure with another method?"
Go through supply: a number of opticians times a number of sales per day per shop. Compare the two results.
"Which assumption would you change first?"
Point to the one that is most uncertain and weighs most on the result, and say how you would refine it.
Frequently asked questions
Do I need to know figures by heart?
A few reference points are enough: the population of France and of the major countries, the number of households, life expectancy, the rough split by age. The rest is built by reasoning, and that reasoning is what gets scored.
Is it better to start from demand or from supply?
Start from demand when the product concerns a broad population you can segment, and from supply when the number of outlets or production capacity is easier to estimate. Using the other method as a check is always appreciated.