Estimate Annual Passenger Journeys on London's Tube Network

How would you work out how many trips people take on the London Underground, the city's subway network, over one year? You have no data in the room, so build the estimate out loud: state each assumption, do the arithmetic, and explain why you believe the result. ```hint Define the unit, then split the demand Agree first on what counts as one trip. Then break the people riding the network into groups whose travel habits you can reason about separately, and look for a second, independent way to reach the number so you can check the first. ``` ### Clarifying Questions - Does a trip mean one passenger journey from entering the network to leaving it, or one train run along a line? - If a passenger changes lines partway through, is that one trip or two? - Should the estimate describe a normal year, or a specific year affected by disruption such as strikes or a pandemic? - Is it the Underground alone, or also the other rail services that share some of its stations? ### What a Strong Answer Covers - A precise definition of a trip, agreed before any numbers. - A clear equation built from segments (for example commuters, other residents and visitors) or from station entries. - Round-number assumptions, each labeled as an assumption, including weekday versus weekend and holiday effects. - Correct arithmetic and a cross-check using an independent method. - A sanity check of the result against the size of the city, plus how real ridership data would replace the estimate. ### Follow-up Questions - If you had the operator's ticket-gate data, how would you count trips exactly, and which journeys would that count still miss? - How would your answer change if the interviewer meant train runs rather than passenger journeys? - Which single assumption moves your answer the most, and how would you narrow it down?