The big idea: Fidelity is how closely a prototype resembles the finished product — in appearance, in materials, and in whether it actually works.
The guide names two techniques used in iterative design: low fidelity and high fidelity.
Low fidelity
- Cardboard and tape mock-up of a ticket machine
- Paper sketches of each app screen, tapped through by a user
- Quick, cheap, easy to throw away
High fidelity
- A kettle prototype in the final plastic, painted, with a working switch
- A clickable app on a real phone that looks finished
- Slow, costly, close to the real product
Two curves decide when to use which. Step through to the second one — willingness to change — which is the one students miss.
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Low fidelity
- Fast and cheap — many ideas tested in a day
- Users criticise it honestly, because it obviously is not finished
- Nobody defends a sketch, so bad ideas die quickly
- But: cannot test function, materials or real ergonomics
- But: some users cannot judge an idea from a rough model at all
High fidelity
- Tests function, materials, weight and real use
- Convinces clients, investors and manufacturers
- Users respond to it as they would to the real product
- But: slow and expensive, so you make very few
- But: everyone becomes reluctant to change something that looks finished
The disadvantage nobody writes down: A high-fidelity prototype takes three weeks to make. When testing finds a flaw, the team that made it argues for a patch rather than a rethink — and a bad design survives because changing it now feels wasteful.
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Which one, when
Still deciding WHAT to make
Low fidelity. Sketches and card models. You will change your mind many times and each change must cost almost nothing.
Concept chosen, checking the shape
Rising fidelity. A block model gives size, proportion and weight you can hold, without any working parts.
Questions become about function and materials
High fidelity. Only a working prototype in the real material answers whether the catch holds or the handle overheats.
Before committing to tooling
Highest fidelity. A pre-production prototype made the way the product will be made, to find what only manufacture reveals.
Iterative means both: The guide calls these techniques used in iterative design. You do not choose one for the whole project — you move up the ladder as the questions change, and every rung is a loop of make, test, refine.
How this is tested — choosing a prototype fidelity for a stage of development and justifying it. It comes up two ways:
Paper 1 — multiple choice
- Identify a stated prototype as low or high fidelity.
- Pick the fidelity that suits a stated stage.
Paper 2 — analysing a product
- Justify the fidelity used at a stage of a named product's development.
- Explain a disadvantage of high-fidelity prototyping.
The trap: Treating high fidelity as simply better. It is slower, dearer, and it makes everyone — including the users — reluctant to change the design. Those are marks if you write them.
A team designing a new kitchen tap has ten weeks. Justify the prototype fidelity it should use in week 2 and in week 8.
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