Also known as: Keystroke-Level Model
The Keystroke-Level Model (KLM) is the simplest variant of GOMS. It models a task as a sequence of physical operators and mental preparations, each with a fixed time estimate. The total task time is the sum of the operator times.
Standard KLM operators and times:
| Operator | Description | Time |
|---|---|---|
| K | Keystroke or button press | 0.20 s (average typist) |
| P | Point to target with mouse | 1.10 s |
| B | Mouse button press or release | 0.10 s |
| H | Home hands (keyboard to mouse or back) | 0.40 s |
| M | Mental preparation | 1.35 s |
| R(t) | System response time | t seconds |
The mental operator (M) is the most important and the hardest to place. Card, Moran, and Newell provided heuristic rules: insert M before each action that initiates a command, before each point-selection of a command, and delete M when the next operator is fully anticipated or within a cognitive unit.
KLM is most valuable for comparing alternative designs for the same task. Absolute predictions may be imprecise, but relative comparisons are usually reliable. Because the M operator (1.35 s) is often the largest single component, reducing mental preparations — through better defaults, clearer labels, or predictable layouts — typically offers the greatest time savings.
KLM can be applied in minutes with nothing more than a task description and arithmetic, making it the most practical GOMS variant.
Related terms: GOMS, Model Human Processor, Fitts's Law, Cogulator
Discussed in:
- Chapter 7: GOMS and the Keystroke-Level Model — The Keystroke-Level Model
- Chapter 17: Predictive Modelling — KLM Analysis in Practice
Also defined in: Textbook of Usability