Page 58 - Safety Risk Management for Medical Devices

Usability Engineering and Risk Analysis 37

3. The User executes the task erroneously. Reasons:
• Perception error
Example: User cannot read a display due to: font size, contrast, brightness,
layout, etc.
• Cognition error
• Memory failure
2 Inability to recall knowledge which was gained before.
2 Lapse. User has the knowledge but temporarily forgets, or omits
an action.
• Rule-based failure
2 Incorrect application of a rule to the task.
2 Application of an inappropriate rule to the task.
• Knowledge-based failure
2 User does not have the knowledge, e.g., due to no training, or not
reading the instructions for use.
2 User applies the wrong mental model, e.g., improvisation under
unusual circumstances
• Attentional error
2 Slip. User has the knowledge and the intention, but executes
incorrectly, e.g., transposing two keys on a keyboard.
2 Physical Error, e.g., pressing the wrong button; reason: buttons are
too small and too close to each other. Or, double hitting of a but-
ton on a UI, e.g., intending to program a dose on an infusion
pump by pressing 4, but accidentally pressing 44 and getting the
wrong dose.

8.5 ENVIRONMENTAL FACTORS

Environmental factors are a contributing Cause to Use Failures. Some environmental
factors that could play a role in causing use failure are:
• Temperature
• Humidity
• Vibration
• Atmospheric conditions
• Distractions
• Lighting
• Physical surroundings
• Acoustic noise
• Workload-related stress
• Other systems and devices in the environment of use