Laboratory 4

Universal Principles, Rules and Laws

Essential principles of interaction design: Consistency, Perceivability, Learnability, Predictability, Feedback, Visibility, Non-Destructive, Discoverability, Reliability.

  • 7 +/- 2 Principle: Short term memory can retain only about 5-9 chuncks of random information at one time (George A. Miller's studies);
  • 2-Second Rule: Users shouldn't need to wait more than 2 seconds for certain types of system response;
  • 3-Click Rule: Users stop using the site if they aren't able to find the information or access the site feature within 3 mouse clicks;
  • 80/20 Rule (Pareto Principle): Approximately 80 percent of the effects generated by any large system are caused by 20 percent of the variables in that system (read more about: The 80/20 Principle by Richard Koch);
  • Stroop Effect this effect in psychology is a demonstration of interference in the reaction time of a task.

Fitts' Law

It predicts the time required to rapidly move to a target area, as a function of the distance to the target and the size of the target.

T = a + b log 2 ( 1 + D W )


  • T is the average time taken to complete the movement;
  • Constants can be determined experimentally by fitting a straight line to measured data:
    • a represents the start/stop time of the device (intercept);
    • b stands for the inherent speed of the device (slope);
  • D is the distance from the starting point to the center of the target;
  • W is the width of the target measured along the axis of motion.

Hick's Law

It describes the time it takes for a person to make a decision as a result of the possible choices he or she has.

T = b log 2 ( n + 1 )


  • T is the average reaction time required to choose among n choices;
  • n equally probable choices;
  • b a constant that can be determined empirically by fitting a line to measured data.

Steering Law

It describes the time required to navigate, or steer, through a 2-dimensional tunnel.

T = a + b C d s W ( s )


  • T average time to navigate through the path;
  • C is the path parameterized by s;
  • W(s) is the width of the path at s;
  • a and b are experimentally fitted constants.

Gestalt Principles

Gestalt ("Shape or Form") psychology describes the way we perceive the world as meaningful and complete objects, not a series of independent parts.

  • Figure/Ground - Elements of a image/object are perceived as either figure (the element of focus) or ground (background on which the figure sits);
  • Prägnanz - People will perceive and interpret ambiguous or complex images/objects as the simplest form possible.;
  • Proximity - Objects that are near one another in space or time are perceived as belonging together;
  • Similarity - Things that are similar or have certain similar attributes (color, size, orientation, texture, font, shape, etc.) are perceived to be more related than things that are dissimilar;
  • Common Fate - Objects moving together are perceived as belonging together;
  • Symmetry - We perceive symmetric objects as figures over ground, and we perceive those objects as symmetric around a central axis;
  • Continuity - Objects aligned along a line or curve are perceived as belonging together;
  • Closure - Incomplete figures are perceived as a complete or whole. When looking at a complex arrangement of individual elements, we tend to look for a single, recognizable pattern.

a) - Closure; b) - Similarity; c) Proximity; d) Continuity; Image provided by

Case Studies

fitts law demostrantion

Fitts' law is usually applied to the movement of the mouse visitors have to perform to get from point A to point B. Fitts' Law Demonstration

Task Analysis & Goals

"Task analysis is the analysis of how a task is accomplished, including a detailed description of both manual and mental activities along with task duration, frequency, allocation, complexity and environmental conditions, necessary clothing and equipment, and any other (unique) factors involved in or required for one or more people to perform a given task." - [source]

  1. Who is going to use system?
  2. What tasks do they now perform?
  3. What tasks are desired?
  4. How are the tasks learned?
  5. Where are the tasks performed?
  6. What’s the relationship between user & data?
  7. What other tools does the user have?
  8. How do users communicate with each other?
  9. How often are the tasks performed?
  10. What are the time constraints on the tasks?
  11. What happens when things go wrong?

Understanding users' tasks a list of questions - [source]

Users want to achieve their goals. Tasks are the process to achieve the goals and should not get in the way of the goals.

  1. False Goals - common misconceived designer goals, because they are easily achieved;
  2. Corporate Goals - important to the corporation, but not necessarily the employees;
  3. Practical Goals - bridge the gap between corporate and user goals;
  4. Personal Goals - true for everyone;