Laws of UX

Chapter 4. Miller’s Law

The average person can keep only 7 (± 2) items in their working memory.

Key Takeaways

• Don’t use the “magical number seven” to justify unnecessary design limitations.

• Organize content into smaller chunks to help users process, understand, and memorize easily.

• Remember that short-term memory capacity will vary per individual, based on their prior knowledge and situational context.

Overview

It’s likely that many designers will have heard of Miller’s law, but there is also a high probability that their understanding of it is inaccurate. This commonly misunderstood heuristic has frequently been cited as justification for design decisions such as “the number of navigation items must be limited to no more than seven” and so forth. While there is value in limiting the number of options available to users (see Chapter 3), it is misleading and inaccurate to attribute such dogma to Miller’s law. In this chapter, we’ll explore the origins of Miller’s “magical number seven” and the real value Miller’s law has to provide UX designers.

Origins

Miller’s law originates from a paper published in 1956 by cognitive psychologist George Miller titled “The Magical Number Seven, Plus or Minus Two: Some Limits on Our Capacity for Processing Information.”¹ Miller, a professor at Harvard University’s Department of Psychology, discussed in his paper the coincidence between the limits of one-dimensional absolute judgment and the limits of short-term memory. Miller observed that memory span in young adults was approximately limited to 7, regardless of the stimuli consisting of vastly different amounts of information. This led him to the conclusion that bits, the basic unit of information, don’t affect memory span as much as the number of information chunks being memorized. The term “chunks” in cognitive psychology refers to collections of basic familiar units that have been grouped together and stored in a person’s memory.

Miller’s paper is often interpreted as arguing that the number of objects an average human can hold in short-term memory is 7±2. Miller himself only used the expression “the magical number seven” rhetorically and was surprised by its frequent misinterpretation. Later research on short-term memory and working memory revealed that memory span is not a constant even when measured in “chunks.”

PSYCHOLOGY CONCEPTChunking

Miller’s fascination with short-term memory and memory span centered not on the number seven, but on the concept of chunking and our ability to memorize information accordingly. He found that the size of the chunks did not seem to matter—seven individual words could be held in short-term memory as easily as seven individual letters. While there are factors that influence how many chunks a given individual can retain (context, familiarity with the content, specific capacity), the takeaway is the same: human short-term memory is limited, and chunking helps us retain information more effectively.

When applied to UX design, chunking informs an incredibly valuable approach to content. When we chunk content in design, we are effectively making it easier to comprehend. Users can then scan the content, identify the information that aligns with their goal(s), and consume that information to achieve their goal(s) more quickly. By structuring content into visually distinct groups with a clear hierarchy, we can align the information we present with how people evaluate and process digital content. Next, we’ll take a look at a few ways this can be achieved.

Examples

The simplest example of chunking can be found in how we format phone numbers. Without chunking, a phone number would just be a long string of digits—significantly more than seven—making it difficult to process and remember. A phone number that has been formatted (chunked) is much easier to both process and memorize (Figure 4-1).

Figure 4-1. A (US) phone number with and without chunking applied

Let’s move on to a slightly more complex example. When browsing the web, you’ll inevitably be faced with the dreaded “wall of text” (Figure 4-2)—content that’s characterized by a lack of hierarchy or formatting and that exceeds the appropriate line length. It can be compared to the unformatted phone number example just given, but on a larger scale. This content is more difficult to scan and process, which has the effect of increasing the cognitive load on users.

Figure 4-2. “Wall of text” example (source: Wikipedia, 2019)

When we compare this example with content that has formatting, hierarchy, and appropriate line lengths applied, the contrast is significant. Figure 4-3 is an improved version of the same content. Headings and subheadings have been added to provide hierarchy, whitespace has been used to break the content into discernable sections, line length has been reduced to improve readability, text links have been underlined, and key words have been highlighted to provide contrast with the surrounding text.

Figure 4-3. “Wall of text” improved with hierarchy, formatting, and appropriate line lengths (source: Wikipedia, 2019)

Now let’s take a look at how chunking is applied in a broader context. Chunking can be used to help users understand underlying relationships by grouping content into distinctive modules, applying rules to separate content, and providing hierarchy (Figure 4-4). Especially in information-dense experiences, chunking can be leveraged to provide structure to the content. Not only is the result more visually pleasing, but it’s also more scannable. Users who are skimming the latest headlines to determine which is worthy of their attention can quickly scan the page and make a decision.

Figure 4-4. Example of chunking applied to dense information (source: Bloomberg, 2018)

While chunking is incredibly useful for bringing order to information-dense experiences, it can be found in many other places as well. Take, for example, ecommerce websites such as Nike.com (Figure 4-5), which uses chunking to group information related to each product. While individual elements might not share a background or surrounding border, they are visually chunked by their proximity to one another (product image, title, price, type of product, and finally total colors available). Additionally, Nike.com leverages chunking to group related filters in the lefthand sidebar.

Figure 4-5. Chunking is commonly used to group products and filters on ecommerce websites (source: Nike.com, 2019)

These examples demonstrate how chunking can be used to visually organize any content for easier comprehension. It helps those who are consuming the content to understand the underlying relationships and information hierarchy. What chunking does not do is dictate a specific limit on the number of items that can be shown at a given time or within a group. Rather, it’s simply a method for organizing content that makes it easier to quickly identify important information.

KEY CONSIDERATIONThe Magical Number Seven

Miller’s law is occasionally misunderstood to mean that there is a specific limit to the number of items that can be stored and processed in short-term memory at one time (7±2), and therefore the number of related interface elements should be limited to this range. A common example in regard to which this law is mistakenly cited is that of adjacent elements such as navigation links. Perhaps you’ve heard someone mention in the past that navigation links must be limited to seven, citing Miller’s law as the justification. In reality, design patterns such as navigation menus don’t require people to memorize them—the choices available to them via the navigation menu are visible at all times. In other words, there’s no usability gain provided by limiting these links to a specific number. As long as the menu is designed effectively, users will be able to quickly identify the relevant link—the only memorization necessary is what their actual goal is.

We’ll return to Nike.com and examine the primary navigation menu (Figure 4-6) to see an example. As you can see, the navigation links number well beyond seven, yet it is still easy to scan the list thanks to clear categorization and the use of whitespace and vertical dividers to separate subgroupings.

Figure 4-6. Despite not limiting menu items to seven, the navigation on Nike.com is easily comprehensible (source: Nike.com, 2019)

Miller’s findings were centered around the limitations of short-term memory and how it can be optimized by organizing bits of information into meaningful chunks. The actual limit to the number of chunks that can be stored will vary per individual based on their knowledge of the information and the complexity of the information. There is research that suggests the average limit closer to four items for short-term memory capacity,² and other prominent theories argue against measuring capacity in terms of a fixed number of elements altogether.³

Conclusion

The sheer volume of information around us is growing at an exponential rate—but we humans have a finite amount of mental resources available to process that information. The inevitable overload that can occur has a direct effect on our ability to complete tasks. Miller’s law teaches us to use chunking to organize content into smaller clusters to help users process, understand, and memorize easily.

¹ George A. Miller, “The Magical Number Seven, Plus or Minus Two: Some Limits on Our Capacity for Processing Information,” Psychological Review 63, no. 2 (1956): 81–97.

² Nelson Cowan, “The magical number 4 in short-term memory: A reconsideration of mental storage capacity,” Behavioral and Brain Sciences 24, no. 1 (2001): 87–114.

³ Wei Ji Ma, Masud Husain, and Paul M. Bays, “Changing concepts of working memory,” Nature Neuroscience 17, no. 3 (2014): 347–356.