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