INFO2222 · Computing 2 Usability and Security
Human Factors and Accessibility
Week 5 covers the human capabilities that constrain interface design — perception, memory and motor performance — plus accessibility and inclusive design, and closes the usability half by linking it to security. The exam tests these as concepts and design implications (recognition over recall, Fitts' law qualitatively, WCAG's POUR) rather than numbers: it will not ask you to write the Fitts' law equation or recall specific frequencies.
What this chapter covers
- 01Cognition and attention: making information salient (colour, grouping, spacing) and avoiding clutter
- 02Perception: foveal (central, in-focus) vs peripheral (blurred, motion-sensitive) vision; pre-attentive 'pop-out' vs attentive serial search
- 03Memory: recognition is easier than recall — prefer menus/icons over recalled commands; reduce cognitive load
- 04Fitts' law as a CONCEPT: pointing time falls as targets get larger and closer; edges/corners act as large targets (equation not examined)
- 05Weber's law: the just-noticeable change is a constant ratio of the baseline stimulus (feedback must scale with baseline)
- 06Accessibility and colour: ~8% of males and ~0.5% of females are colour-blind — never encode meaning by colour alone
- 07Microsoft inclusive design: disability as a person-environment mismatch (permanent/temporary/situational); solve for one, extend to many
- 08WCAG POUR principles — Perceivable, Operable, Understandable, Robust — and the usability-and-security intersection
Apply Fitts' law and recognition-over-recall to a toolbar
- +1(a) Layout Y is faster. Fitts' law says selection time falls as the target's width increases and its distance decreases; the large button is a bigger target, and pinning it to a screen edge makes it effectively very large because the pointer cannot overshoot past the edge.
- +1Reinforce the design implication: make frequently used controls larger and place them near the pointer or at edges/corners, which behave as large targets — exactly what layout Y does. (No equation is needed; reason qualitatively.)
- +1(b) Replacing icons with typed command names is a poor memory choice. Recognition is easier than recall, so icons and menus (which users recognise) reduce cognitive load, whereas recalling exact command names loads memory and raises errors.
- +1(c) One limitation: Fitts' law predicts only expert, error-free performance and ignores age, disability and mood (it also needs expertise/time to model). Any one of these is acceptable.
Key terms
- Foveal vs peripheral vision
- Foveal vision is the small central region in sharp focus; peripheral vision is the blurred outer field used mainly to detect movement and brightness change. Interfaces should place fine detail centrally and use the periphery for alerts to motion.
- Pre-attentive vs attentive processing
- Pre-attentive perception processes certain features (colour, shape, texture) in parallel, so an item differing in one feature 'pops out' in under about a quarter of a second. Attentive perception is slow, serial, effortful scanning. Good layouts exploit pop-out to make targets salient.
- Recognition over recall
- A memory design principle: recognising a shown option (a menu item, an icon, a browser-history entry) is easier than recalling it from memory (a command name). Interfaces should let users recognise rather than remember, and should reduce cognitive load.
- Fitts' law (concept)
- A model of pointing: the time to acquire a target falls as the target gets wider and closer. Design implication — make frequent controls larger and nearer, and use edges/corners as effectively large targets. It predicts only expert, error-free performance; the exam does not require the equation.
- Inclusive design
- Microsoft's view that disability is a mismatch between a person and their environment and is context-dependent (permanent, temporary or situational, e.g. one arm / an arm injury / holding a baby). Principles: recognise exclusion, learn from diversity, solve for one and extend to many.
- WCAG POUR
- The four Web Content Accessibility Guidelines principles: Perceivable (e.g. alt text, captions, colour contrast), Operable (e.g. keyboard access), Understandable (e.g. readable, predictable), Robust (interpretable by assistive technologies).
Human Factors and Accessibility FAQ
Do I need to memorise the Fitts' law equation or receptor frequencies?
No. The revision material is explicit that you will not be asked to write the Fitts' law equation, calculate statistics, or recall specific numbers, angles or frequencies (such as mechanoreceptor ranges). What you do need is the concepts and their design implications — larger, closer targets are faster; recognition beats recall; feedback should scale with the baseline stimulus (Weber's law). Confirm the exact scope in the Week 12 revision on Canvas.
Why shouldn't information be encoded by colour alone?
Because roughly 8% of males and 0.5% of females have some colour blindness, so meaning carried only by colour can be lost. Add a second cue — text, shape, position or pattern — and choose colour-blind-safe pairs (yellow on black reads well; yellow on white or green is poor). Accessibility fixes like this are often low-effort and benefit everyone, echoing the inclusive-design idea of solving for one and extending to many.
How does usability actually help security?
The interface can strengthen security in several ways: user-centred design that still blocks bots (reCAPTCHA), clear warnings (a browser 'your connection isn't private' notice), alternative paths (granular cookie-consent choices), and design that mitigates vulnerabilities (masking password fields, reducing shoulder-surfing). The theme is balance — overly strict rules (very long, symbol-heavy passwords) hurt usability, so there is always room to design for both. This bridge sets up the security half of the unit.
Can AI help me with human factors and accessibility in INFO2222?
Yes, as a study aid. Sia can quiz you on perception and memory principles, walk through a Fitts' law design implication qualitatively, map a fix to the right WCAG POUR principle, and explain inclusive-design ideas. Use it to rehearse the concepts and design reasoning for the quizzes and your project; it does not complete graded work for you, and the University of Sydney academic-integrity policy applies.
Exam move
Study Week 5 as concepts-plus-implications, never as numbers. For each human capability, learn one design rule you can state fast: perception → make targets salient and exploit pop-out; memory → recognition over recall and reduce cognitive load; motor → Fitts' law means bigger and closer (edges/corners are large targets); sensation → Weber's law means feedback scales with baseline. For accessibility, memorise the WCAG POUR principles with one example each, the colour-blindness figures (~8% male, ~0.5% female) as a 'don't rely on colour' rule, and the inclusive-design 'solve for one, extend to many' line. Rehearse the usability-and-security bridge examples, since they connect the two halves the exam samples equally. Skip drilling receptor frequencies and the Fitts' equation — they are explicitly out of scope. Confirm on Canvas.
Working through Human Factors and Accessibility in INFO2222? Sia is AskSia’s AI Computer Science tutor — ask any INFO2222 Human Factors and Accessibility question and get a clear, step-by-step explanation grounded in how INFO2222 is taught and assessed. Read this chapter free, then take your hardest questions to Sia.
