The big idea: "Usable" is not one thing: a product can pass on some usability objectives and fail badly on others.
The guide's five objectives
- Learnability
- Efficiency
- Memorability
- Errors
- Satisfaction
Why separate them: Separating them is what lets an evaluation say WHICH kind of usability is failing, and therefore what to change.
All five applied to one product. For each, the middle row is the one students skip — what you would actually measure.
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Learnability and efficiency pull against each other: A machine that walks a first-timer through one step per screen is highly learnable and infuriating to a user who does it daily.
One that fits everything on a dense screen is fast for regulars and unusable for a first visit. Which you favour depends on who your primary persona is.
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| Objective | The question it asks | A measurement you could take |
|---|---|---|
| Learnability | Can a first-time user finish without help? | Of 20 first-timers, how many completed the task unaided |
| Efficiency | Once learned, how quickly can it be done? | Seconds from start to finish for a practised user |
| Memorability | Can a returning user still do it after weeks? | Errors on a second attempt four weeks later |
| Errors | How many, how serious, how recoverable? | Mistakes per 100 uses, and how many could be undone |
| Satisfaction | How pleasant was it? Would they choose it again? | A 1-5 rating, plus what users say while waiting |
An evaluation with no measurement is an opinion: The command term in this statement is evaluate. "The machine is not very learnable" is an opinion and earns little.
"Only 6 of 20 first-time users paid without asking someone for help" is an evaluation, and it tells you how much better the redesign has to be.
Errors has three parts: Count them, weigh them, and check whether they can be undone.
An error you can correct in one tap barely matters. An error that charges the wrong card and cannot be reversed matters enormously, even if it happens rarely.
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The statement says usability objectives are used to identify opportunities for improvement. That is the second half of the work: every poor measurement has to become a specific change to the product.
Working from a failing objective to a change
Name the objective that failed
Not "it is hard to use". Learnability failed: 6 of 20 first-timers finished unaided.
Find where in the task it failed
Which step did they stop at, and what did they try instead? This is where a task analysis and a usability evaluation meet.
Name the feature responsible
The card reader is below the screen, out of the light, and nothing points to it. That is a feature, and features can be changed.
State the change and how you would re-test
Move the reader to the right of the screen with a lit surround, then run the same 20 first-timers test and compare the number.
Re-measure the same way: The improvement is only proved if you measure it with the same method and the same users' characteristics as before.
A redesign tested on your classmates after the original was tested on real visitors proves nothing at all.
How this is tested — evaluating a product against the five usability objectives and turning failures into changes. It comes up two ways:
Paper 1 — multiple choice
- Match a described failure to the objective it belongs to.
- Pick the measurement that tests a stated objective.
Paper 2 — analysing a product
- Evaluate a named product against three usability objectives.
- Explain one improvement that a failed objective points to.
The trap: Using the five objectives as five synonyms for "easy to use". They are different questions with different measurements and different fixes — and an answer that treats them as one scores once, not three times.
A supermarket self-service checkout is criticised by customers. Evaluate it against three usability objectives, and identify one improvement for each.
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