KLM, GOMS, and Predictive Modelling | Interaction Design

The Keystroke-Level Model (KLM)

KLM predicts the time an expert user takes to complete a well-defined, error-free task by summing unit operator times:

Operator	Symbol	Typical Time	Description
Keystroke / button press	$K$	~200 ms	Pressing a key or tapping a button. Varies by typing skill.
Pointing	$P$	Fitts’ Law	Moving the cursor or finger to a target. Estimated via Fitts’ Law, not a fixed constant.
Homing	$H$	~400 ms	Moving the hand between mouse and keyboard.
Mental preparation	$M$	~1350–1500 ms	The cognitive preparation before a chunk of automated action — deciding what to do next.
System response	$R$	Depends on system	Waiting for the computer to respond.

Total predicted time $= \sum$ (operator times). The analyst decomposes the task into a sequence of these operators, sums the times, and compares across alternative designs.

The M-Operator Placement Heuristics

The mental operator $M$ is the trickiest to place — it represents a conscious cognitive chunk. Standard heuristics for inserting $M$ operators:

Before every chunk of actions that form a single cognitive unit.
Before every decision point (choosing between methods).
Not between actions that are part of the same automated motor sequence (e.g. typing a memorised word — one $M$ before the word, not one per letter).

Worked KLM Example

For the task “save the current document using Ctrl+S”:

$M$ (decide to save): 1350 ms
$K$ (Ctrl key): 200 ms
$K$ (S key): 200 ms
Total: 1750 ms

For the same goal using the mouse: File menu → Save:

$M$ (decide to save): 1350 ms
$H$ (hand to mouse): 400 ms
$P$ (point to File menu): estimated via Fitts’ Law
$K$ (click File): 200 ms
$P$ (point to Save): via Fitts’ Law
$K$ (click Save): 200 ms
Total: substantially longer, dominated by homing and pointing time.

This comparison explains why keyboard shortcuts are faster for experts — they eliminate homing and pointing operators, at the cost of requiring recall (the user must remember the shortcut, which KLM does not model).

GOMS: Goals, Operators, Methods, Selection Rules

GOMS generalises KLM into a full cognitive architecture:

Component	Description
Goals	What the user wants to accomplish — the top of the hierarchy.
Operators	The atomic actions available to the user — keystrokes, mouse movements, mental operators (the same set as KLM).
Methods	Learned procedures — sequences of operators — for achieving a goal. An expert has multiple methods for the same goal.
Selection Rules	When more than one method exists, rules for choosing between them. “If the hand is already on the mouse, use the menu. If on the keyboard, use the shortcut.”

GOMS is used to predict expert task-completion time and to compare alternative interface designs before either is built, provided the task is well-defined, repetitive, and performed by an expert executing a known method.

Three (Plus One) Limitations of GOMS/KLM

These limitations are highly examinable (2018 Q5c directly asks for three):

Expert-only assumption: GOMS/KLM assumes the user already knows the optimal method — it says nothing about learnability, novice behaviour, exploration, or errors. A novice who does not know the shortcut exists will use the slower menu method, and KLM cannot predict this.
No error modelling: real usage includes mistakes, hesitation, recovery, and backtracking — none of which GOMS/KLM captures. A system that is theoretically faster (in error-free expert use) but more error-prone may be slower in practice.
Requires a fully-specified, repetitive task: unsuitable for exploratory, creative, or highly variable activities (cf. Cognitive Dimensions, which was designed for precisely this situation). If the user’s goal is not known in advance, the method cannot be decomposed.
Mental preparation time ( $M$ ) is a coarse average: it varies substantially by individual, task familiarity, fatigue, and context, reducing predictive precision for any one user in any one session. The model gives a population-level estimate, not an individual-level prediction.

Expected Learning

State the five KLM operators ( $K, P, H, M, R$ ) with typical times.
Decompose a described task into KLM operators and estimate total time.
Explain why keyboard shortcuts are faster than menu navigation in KLM terms.
Define GOMS and its four components (Goals, Operators, Methods, Selection Rules).
Name and explain three limitations of GOMS/KLM, linking each to what the model cannot predict.
Compare GOMS/KLM with Cognitive Dimensions: when is each the appropriate analysis tool?

Past Paper Questions

GOMS/KLM appears in: 2018 Q5(c) (explain GOMS, list steps, describe KLM, three limitations). KLM may appear in any question requiring comparison of two interaction methods for an expert user performing a repetitive task.