Basics of Human Perception: Short-Term Memory

We build applications for people. That’s why human perception is the foundation of every good design. Various insights from psychology make it easier for us to present information so it’s easy to understand, and to arrange elements so they’re easy to grasp. To better understand human perception, we’ve launched a multi-part series. We’re starting with short-term memory:

The 7 Plus or Minus 2 Rule of Short-Term Memory:

Human memory can be divided into 3 parts: sensory memory, short-term memory, and long-term memory. These 3 parts of memory have different properties, which is why they’re treated as distinct systems of our memory. We won’t go into long-term memory in more detail here, since we don’t deal with it in usability.

Our sensory memory holds items for about 1 second before they either pass into short-term (or working) memory or are discarded. Short-term memory stores content for a variable length of time. The duration varies because our attention keeps turning to new information that needs to be stored in short-term memory. Compared to sensory memory or long-term memory, the capacity of short-term memory is much smaller.

“There is a clear and definite limit to the accuracy with which we can identify absolutely the magnitude of a unidimensional stimulus variable. I would propose to call this limit the span of absolute judgment, and I maintain that for unidimensional judgments this span is usually somewhere in the neighborhood of seven. We are not completely at the mercy of this limited span, however, because we have a variety of techniques for getting around it and increasing the accuracy of our judgments” (Source: http://www.well.com/user/smalin/miller.html )

In 1956, the American psychologist George Armitage Miller described the fact that a person can only process 7 plus or minus 2 units of information (chunks) in short-term memory at the same time. The size of short-term memory is genetically determined and can’t be increased through training.

So what does this mean for me as a UX/UI designer?

Human short-term memory can’t process more than 7 units of information. That’s why we keep the amount of information on a page as low as possible and combine related units of information into chunks. This keeps the cognitive load low. In an online shop, we offer a maximum of 5 different pricing options. And the navigation doesn’t hold more than 5–7 items either.

The Paradox of Choice:

The phenomenon known as the “paradox of choice” was first described by the American psychologist Barry Schwartz in his book of the same name. Schwartz argues that too much information overwhelms us. If there are 24 different kinds of jam in front of us, we have an extremely hard time choosing one. But if the store only has 3 kinds of jam, the decision is much easier. We can apply this principle to human-computer interaction: on the one hand, we want as much choice as possible; on the other hand, we’re more likely to decide on something when the selection is smaller and it’s easier for us to work out which option is best. Remember: short-term memory only processes 7 plus or minus 2 items at a time. If our selection is larger, the cognitive load increases. Users need to find a solution quickly to reach their goal. But if they first have to work their way through a bunch of variations, their short-term memory stays overloaded the whole time.

Looking Ahead:

That was the first part of the series “Basics of Human Perception.” In it, we looked at short-term memory and the “paradox of choice.”