Five HCI Laws for User Experience Design

Jeff Sauro, PhD

Usability is hardly physics or chemistry.

But there are some important principles from decades of research in Human Computer Interaction (HCI) that apply to design and user research.

Here are five famous laws that can be applied to improving the user experience of applications and websites:

  1. Miller’s Law of Short Term Memory Load: The psychologist George Miller described a variety of phenomenon which suggested that most people can only hold approximately seven pieces of information in their short term memory at once (the Magic Number 7+- 2).  For example, most people can remember a random seven-digit phone number.  More information can be stored by “chunking” information into meaningful groups and to hold the information in memory, people have to be able to focus on rehearsing it (mentally “speaking” it) .  Caution: This law should not be applied to things like the appropriate number of items in a menu or a selection list because the selection task does not require people to hold the options in memory.
  2. Fitts’ Law: Paul Fitts described a way to mathematically predict how long it will take to “acquire” a target based on its distance and size. When applied to interface design, this law means that it takes users longer to point to links and buttons on a screen if the objects are smaller in size or farther away from the home position. It’s a cornerstone of Keystroke Level Modeling which helps you predict how long it will take skilled users to complete tasks.
  3. Hick-Hyman Law: Hick described the time it takes for a person to make a decision based on the number of choices available. People subdivide the total collection of choices into categories, eliminating about half of the remaining choices at each step, rather than considering each and every choice one-by-one, which requires linear time.   Hyman later found that a linear relationship exists between reaction time and the information transmitted. For some recent applications see A Predictive Model of Menu Performance[pdf].
  4. Power Law of Practice: Described by Newell and Rosenbloom, this law states that the time to complete a task decreases linearly with the number of practice trials taken when both are expressed as logarithms. This is where the famous “learning curve” is derived from. If you know absolutely nothing about a topic, you can learn 50% of the information quickly. But when you have 50% less to learn, it takes more time to learn that final 50%.
  5. Pareto and Zipf LawsThe Pareto principle is named after the Italian economist Vilfredo Pareto, who found that a majority of the land in 19th century Italy was owned by the minority of the population. It’s often called the 80/20 rule, and it can be applied to the most profitable customers and most critical usability problems, as well as identifying top tasks. Related to the Pareto law is Zipf’s law, which was named after a linguist who noticed that the most frequent word will occur approximately twice as often as the second most frequent word, three times as often as the third most frequent word, and so on.  It also applies to customers for product types, word frequency in a verbatim analysis and the frequency of commands used in software[pdf] like Microsoft Word.

If you feel the need to brush up logarithms after reading the above laws that show logarithmic relationships, you’re not alone. The logarithm is an important relationship in human behavior, yet it is less intuitive than linear relationships.  Many of these laws are described in the seminal 1983 book, The Psychology of Human Computer Interaction

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