That's too simplified! You can't just take a random # out of the population and call it a day.
Market sizing is about breaking the problem down into logical segments that move you from guessing towards estimating.
I would personally eliminate age groups as you've mentioned, and then segment the population by income. Income is going to dramatically affect who can own a car or not.
How to think about Market Sizing
It's very simple: Do the approach the is the easiest for you given the question.
Are they asking you to estimate something where you don't even know where to begin from the top (maybe you have 0 clue as to the market size of the industry, the GDP of that country, etc. etc.)? Then do bottom-up!
Alternatively, does it seem impossible to do a realistic from-the-ground-up estimation of something (perhaps it requires just far too many steps and assumptions)? Then do top-down!
Fundamentally, you need to take the approach that just makes the most sense in that circumstance. Quickly think about the key assumptions / numbers required and whether you 1) Know them or 2) Can reasonably estimate them. If you can, go ahead!
Remember that there's rarely a "best" answer with market sizing. What's important is that you break down the problem the way it makes sense to you. Importantly, break it down so that the assumptions you make are the ones you're most comfortable in.
For example, do you know all the major brands? Great go with that. Do you understand all the segments of that country's population (either age or wealth or job breakdown)? Go with that. Do you know the total market size of the tourism (or hotel) industry? Then break it down that way.
- Just like in a case, make sure you understand the question - what are you really being asked to calculate
- Decide whether a top-down or bottom-up approach is best
- Figure out what you know you know, and what you know you don't know, but could estimate
- This helps you determine how to split out buckets
- Stay flexible - you can start with a "high-level" market sizing, but gauge your interviewers reaction....if it looks like they want you to do more...then go along level deeper in terms of your splits
Take a look here for additional practice! https://www.preplounge.com/en/management-consulting-cases/brain-teaser/intermediate/taxis-in-manhattan-market-sizing-229
And here's a practice Q&A:
This one could be answered top-down (as I did) by estimating population of the city, # of drivers/ cars, etc. etc.
OR, it could be answered bottom-up by estimating # of stations you see per block (or # of gas/petrol tanks), % increase this might be over time (or # of EV stations that would be needed per gas tank given EV stations take 10 times as long), and # of blocks you'd estimate the city to have.