Usability and HCI

Know your users

perception

cognition

action

Know your users' tasks

Keystroke level modelling

better models available

At least make a scenario

Don't forget extrataskular items, like communication, social

Know your technology

be it HTML

Displays

CPUs

printers

All the rest is details (?)

Keystroke-level modelling

[Taken from an online book by Gilmore, Churchill, & Ritter, Introduction to Human Factors]

also see Card, Moran and Newell, The psychology of human-computer interaction, 1983. Hillsdale, NJ: LEA. A shorter version is available as Card, S. K., Moran, T. P., & Newell, A. (1980). The keystroke-level model for user performance time with interactive systems. Communications of the ACM, 23(7), 396-410.

Also see Gray, W. D., John, B. E., & Atwood, M. E. (1993). Project Ernestine: Validating a GOMS analysis for predicting and explaining real-world task performance. Human-Computer Interaction, 8(3), 237-309. [how they saved a million/year for Nynex]

Revised 9/II/00-FER

When the operators are analysed down to elemental perceptual, motor (e.g. keystrokes), cognitive actions, then by noting where the time goes it can be possible to make time predictions for expert error-free performance.

The execution of a unit-task requires operators of (basically) 4 types:

1. Keystrokes - Unit time based on typing speeds (0.08 - 1.2 s)
2. Homing - moving hand to / from mouse and keyboard (gener 0.4 s)
3. Pointing - moving mouse to target (clicking is a keystroke) (gener. 1.1 s)
4. Drawing - dragging mouse in straight-line segments (0.9 n+0.16 l - n = number of segments, l = length of segments)

To these should be added some number of mental operators (1.35 s),

and if it limits the user's task performance, some estimate of the system's response time.

The number of mental operators comes from a set of rules -- basically between all operators, except those linked through knowledge or skill (e.g. the keystrokes in a single word, or point, click mouse).

Where there are selection rules governing the choice of methods then it is up to the analyst to decide whether to go for best or worst case time predictions.

Requires

• experts
• single task
• practice with the task
• no uncertainty with vision
• No tricky critical path (ie thinking while waiting for system)

Example

Method

1. Delete 3rd clause and H [mouse] PK PK M K [D]
2. Insert it in front of 1st clause. PK M K [l] K [ESC]
3. Replace ": o" with "O". PK M K [R] K [SHIFT] H [keyboard] 2K [O ESC]
4. Replace "T" by ": t". H [mouse] PK M K [R] H [keyboard] 4K [: SPACE t ESC]
5. Delete 3rd clause and H [mouse] PK PK M K [D]
6. Insert it in front of 2nd clause PK M K [l] K [ESC]
7. Find next task. M K [F]

Time Predictions

Texecute = [24tK + 8tP + 5tH ] + 7tM
= [24(0.15) + 8(1.03) + 5(0.57)] + 7(1.35)
= 14.7 + 9.4
= 24.1 s

Commentary

Although closely related to GOMS, note how keystroke-level modelling is really closer to time-and-motion (chronological) analysis than goal/subgoal analysis. It assumes a concentration on one task at a time, no interleaving of goals, no interruptions, a single method, and so on. If the model is off, which it will be, it is typically off equally and commensurately for the two designs being considered, so it is usually fair.

Indeed, KLMs can be achieved without worrying about goals and selection rules.

Quite a considerable effort has gone into trying to make them more usable - particularly by building computer tools to apply them (Nichols & Ritter, 1995; Beard, Smith, & Denelsbeck, 1996). The tools themselves, however, may be like the cobbler's children.

Design a menu now