Fitts' Law

A Demonstration of Human-Computer Interaction and Ergonomics.

About This Demonstration

Fitts' Law (1954) predicts that the time required to rapidly move to a target area is a function of the ratio between the distance to the target and the width of the target.

"The time to acquire a target is a function of the distance to and size of the target."

Instructions:
  1. Click the Dark Blue start circle in the centre.
  2. Immediately click the Red target circle that appears as fast and accurately as possible.
  3. Repeat until the demonstration concludes (12 trials).
Demonstration Stage
Trial: 0/12 Avg Time: 0 ms

Ensure your mouse path is clear.

Analysis & Debrief

Performance Visualisation

Relationship between Index of Difficulty (ID) and Movement Time (MT).

Data Summary
ID (bits) Avg Time (ms)
Interpretation: Steeper slopes indicate lower throughput (slower processing). The linear relationship confirms Fitts' Law.
Clinical & Safety Implications

In healthcare interfaces (e.g., Electronic Health Records, Infusion Pumps, Ventilators), Fitts' Law dictates safety:

  • Critical Actions: Buttons for functions like "Emergency Stop" or "Defibrillate" should be large and close to the 'neutral' position to minimise time delays and error.
  • Dangerous Actions: Buttons for "Delete Record" or "Shut Down" should be smaller or placed further away from 'neutral' to force a deliberate, slower cognitive process.
  • Pop-up Fatigue: Although not specifically demonstrated here, 'Popup Fatigue' is a phenomenon where users, particularly in healthcare settings, become overwhelmed by the sheer volume of pop-up notifications or alerts generated by electronic systems, such as Electronic Health Records (EHRs). This desensitisation can result in users ignoring or bypassing important alerts, which can compromise safety and decision-making.
Real-World Example: The "Confirm" Button

Many medical device errors occur because "Cancel" and "Confirm" buttons are small and placed adjacent to each other. According to Fitts' Law, the risk of "undershooting" a small target (clicking the wrong button) increases significantly as speed increases during an emergency.

References:
Fitts, P. M. (1954). The information capacity of the human motor system in controlling the amplitude of movement. Journal of Experimental Psychology, 47(6), 381–391.
MacKenzie, I. S. (1992). Fitts' law as a research and design tool in human-computer interaction. Human-Computer Interaction, 7(1), 91-139.