Principles of Design #36 – Performance Load

The path of least resistance

In life, it’s inevitably true that the greater the effort to accomplish anything, the less likely it is that it will be accomplished, a truth known by designers as the law of performance load (and also called the ‘path of least resistance’ and the ‘principle of least effort’). For any design, the amount of physical and.or mental effort required to achieve a goal is the performance load, and as it increases, performance time and errors increase and hence the probability of successfully completing a task decreases. Conversely, as the performance load decreases, the task becomes easier and the likelihood of completion increases (and time and errors decrease). There are two types of load which contribute to performance load, related to the mental effort (called cognitive load) and the physical effort (called kinematic load).

Cognitive load is the amount of mental activity required in accomplishing a goal, including perceptual tasks, memory requirements and problem solving. For example, graphical user interfaces have dramatically decreased the cognitive load involved in performing tasks on computers (I am using one as I upload this article through a standard blogging software). In the early days of computers, users had to learn long lists of commands and then type them into the computer in very specific sequences and structures, whereas I am now able to browse from a menu of commands (helpfully broken into easy to understand hierarchies) and choose the one I need to use. A large part of the success of computers as mass market devices is the reduction in cognitive load.

There are a number of strategies that can be used to reduce mental effort, including minimising visual noise (and maximising signal), chunking information that needs to be remembered (as in browser menus), using memory aids (eg icons) to assist recall and problem solving, and automating tasks which make heavy demands on computation and memory.

Kinematic load is the amount of physical activity required to accomplish a goal, and is impacted by the number of steps or movements involved in the task and the amount of force required for each of those steps. When the telegraph was invented, and communication was conducted through a mechanical ‘tapping’ letter by letter, the number of taps required to communicate a message was the kinematic load. Samuel Morse specifically designed Morse Code to minimise this load by keeping the simplest codes for the most common letters (E is a single dot, whereas Q is much longer dash dash dot dash). This design reduced the physical effort required to send messages, significantly reducing transmission times and error rates.

Other strategies for reducing kinematic load include reducing the number of steps required, minimising the range of motion and travel distances (hence time and motion studies!), and automating tasks which are highly repetitive.

Designers and marketers need to minimise performance load as much as possible when designing any interaction with a customer (and that’s not an excuse for complicated call centre menus and messages which say ‘your call is important to us’).

Cognitive load can be reduced by eliminating unnecessary information from displays (reducing noise), chunking information into ‘bite’ sized pieces (for ease of remembering), using memory aids to help in complex tasks (eg icons and symbols) and automating tasks which required significant computation or memory. Physical effort can be reduced by minimising the number of steps in any task, reducing the amount of motion and energy required, and automating repetitive tasks when feasible.


Principles of Design, Revised and Updated by William Lidwell, Kritina Holden and Jill Butler (2010)

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