The big idea: One fixed size fits almost nobody well. There are exactly two strategies for covering a percentile range:
- make one product adjustable - make several fixed sizes
Most real products use both.
Both strategies drawn against the same population bar, so the coverage each buys is comparable rather than asserted.
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Adjustability
- One product covers the whole 5th–95th range
- One set of tooling, one stock line
- The user fine-tunes the fit themselves
- But: a mechanism costs more and can fail
- But: it only works if the user actually adjusts it
A range of sizes
- Each item is simpler — nothing to adjust or break
- Cheaper per unit, lighter, often more durable
- Fit is immediate with no user action
- But: three sets of tooling and three stock lines
- But: the user has to choose correctly, and shops can be out of a size
Which one wins?: It depends on how long the user keeps the product and whether one person uses it.
A shared office chair changes user every day, so it adjusts. Your shoes are yours alone and never change size, so they come in sizes.
A height-adjustable desk can be set between 680 mm and 1180 mm. Calculate its range of adjustability.
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Cycle helmet
Two or three shell sizes, each with an adjustable cradle and chin strap. Sizes get the shell roughly right cheaply; the cradle closes the last gap, which matters because a loose helmet does not protect.
Office chair
Adjustable only — height, back angle, armrests. It is shared, expensive and kept for years, so one adjustable chair beats stocking three sizes of a costly item.
Running shoes
Sizes only. A lightweight sole cannot carry an adjustment mechanism without ruining the thing the product is for, and a shoe has one owner.
Car driver's seat
Adjustable, because a family car has several drivers of very different sizes and swapping the seat is not an option.
Identify, then justify: The statement asks you to identify products that use one or both strategies. Have three or four examples ready, and be able to say why that product went that way — sharing, cost, weight, or how long it is kept.
How this is tested — choosing between adjustability and a range of sizes, and saying why. It comes up two ways:
Paper 1 — multiple choice
- Identify which strategy a described product uses.
- Pick the strategy that suits a stated constraint such as low weight.
Paper 2 — analysing a product
- Explain why a named product is adjustable rather than sized.
- Evaluate the cost and usability trade-off between the two.
The trap: Answering "adjustable is better" in general. It is not — adjustment adds weight, cost and something to break, and a product with one owner rarely needs it.
A school buys chairs for a shared computer room and helmets for its cycling club. Explain the fitting strategy most appropriate to each.
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