Worked Tripos Examples: Full Model Answers | Interaction Design

2019 Paper 3 Q6(c): Gestalt Theory Applied to Four UI Elements

Question: What does Gestalt theory describe and what is its implication for interaction design? Describe which principle(s) are being applied for four given interface elements.

Model answer — definition: Gestalt theory describes how humans perceive visual elements as organised wholes, driven by innate perceptual grouping tendencies, rather than analysing each element independently. The implication for interaction design is that designers can deliberately exploit these grouping tendencies to communicate structure and relationships before the user consciously reasons about the interface — and that misapplying them (placing unrelated items close together) actively misleads users.

Model answer — four elements:

(a) Blog-post creation interface: Proximity groups the title field with the body editor as one authoring unit, visually separated from a distinct “Publish” action area. Similarity — formatting-toolbar icons share a consistent style, signalling they are all text-formatting commands.
(b) Twitter notifications icon: Figure/Ground — a solid-coloured badge sits in front of the bell icon, making the unread count the perceptual “figure” against the bell as “ground.” The user’s attention is drawn to the badge without effort.
(c) Twitter profile layout: Proximity and Similarity group the follower/following count block as related statistics, visually distinct from the bio text above. The layout communicates “these numbers belong together” before the user reads them.
(d) Google Maps walking directions: Continuity — the highlighted route line leads the eye smoothly along the intended path, distinguishing it from the surrounding, unhighlighted street grid. The route is perceived as a single continuous entity.

Exam pattern: always use the three-step method — name → locate → explain effect. Every element gets all three.

2018 Paper 3 Q5(c): GOMS and KLM

Question: Explain what GOMS analysis predicts, list its steps, describe KLM, and explain three limitations.

What GOMS predicts: the time an expert user takes to complete a well-defined, repetitive task, by modelling Goals, Operators, Methods, and Selection Rules.

Steps of a GOMS/KLM analysis:

Define the goal and the expert method used to achieve it.
Decompose the method into unit operators (K, P, H, M, R).
Estimate time for each operator (Fitts’ Law for P; fixed constants for K/H/M; system-dependent for R).
Sum operator times to predict total task time.
Compare predicted times across alternative interface designs.

KLM operators: K (keystroke, ~200 ms), P (pointing via Fitts’ Law), H (homing hand between mouse/keyboard, ~400 ms), M (mental preparation, ~1350–1500 ms), R (system response time).

Three limitations:

Expert-only: assumes the user already knows the optimal method — says nothing about learnability, novice behaviour, or errors.
No error modelling: real usage includes mistakes, hesitation, and recovery — none of which KLM captures. An interface predicted to be faster by KLM may be slower in practice if it is more error-prone.
Requires a fully-specified, repetitive task: unsuitable for exploratory, creative, or highly variable interaction — precisely the domain where Cognitive Dimensions is a better fit.

2018 Paper 3 Q6(c): Fitts’ and Hick’s Law Layout Comparison

Question: Explain what Hick’s Law and Fitts’ Law predict, and use them to compare two candidate home-screen layouts.

Hick’s Law ( $T = b\log_2(n+1)$ ) predicts decision time grows logarithmically with the number of simultaneous choices. A flat menu with many options will be slower to choose from than a hierarchical one, even if the hierarchical one requires an extra navigation action, because $\log_2$ is concave.

Fitts’ Law ( $\text{MT} = a + b\log_2(2D/W)$ ) predicts pointing time from target distance and width — larger, closer targets are faster.

Comparison: if Layout A presents all controls (current location, search, settings, warnings, help) as one flat, widely-spaced list, and Layout B groups them into two categories with tighter spacing, Layout B is predicted to be faster on both laws: fewer simultaneous top-level choices (Hick’s) and shorter average pointing distance (Fitts’).

Caveat: this prediction holds only for an expert user with a learned mental model of the grouping. If Layout B’s grouping does not match users’ natural categorisation (e.g. they expect “Settings” to be separate from “Search”), the extra navigation step could increase search time and errors — a limitation neither law captures because neither models learnability or incorrect mental models.

2020 Paper 3 Q5(d): HE vs CW for a Novel Sensory-Substitution Device

Question: A wearable device converts colour into sound. How would you use Heuristic Evaluation and Cognitive Walkthrough to evaluate it, and how do the two compare?

Heuristic Evaluation is weakened here because Nielsen’s heuristics were derived largely from conventional graphical interfaces (e.g. “match between system and real world,” “consistency with standards”). There is no established convention for how colour-to-sound mappings “should” behave, so evaluators have little to compare against. HE relies on comparing the interface to known conventions; for a genuinely novel paradigm, those conventions do not exist.

Cognitive Walkthrough is comparatively stronger: it requires only a concrete task (e.g. “identify that a wall in front of you is painted red”) and a model of the target user (a visually impaired person experienced in non-visual navigation) — no prior convention is needed, only a clear goal and a walk through whether each step is discoverable and gives feedback.

Conclusion: for genuinely novel interaction paradigms, favour CW. Alternatively, use a hybrid: conduct CW first to surface concrete task failures, then use the surfaced patterns to write bespoke, device-specific heuristics for a lighter, second-pass HE.

2017 Paper 3 Q6(c): Heuristic Evaluation of a Music Player

Question: Indicate which usability principle(s) each of five listed problems violates.

Problem	Primary Heuristic
P1. Inconsistencies between menus and buttons	#4 Consistency and Standards
P2. Language doesn’t match user terminology	#2 Match Between System and the Real World
P3. Unrecognisable buttons (users cannot tell what they do)	#6 Recognition Rather than Recall
P6. Undo unsupported	#3 User Control and Freedom
P8. Insufficient status information during a task	#1 Visibility of System Status

Exam note: some problems can violate more than one heuristic. P3 (unrecognisable buttons) could also be read as #8 (Aesthetic and Minimalist Design) if the buttons were added purely for visual style without function. Always state your primary heuristic first, then note a secondary one only if you can justify it concretely with reference to the scenario.