Chapter 88. Short-Term Memory Capacity

1. Information in our short-term memory (STM) is very fragile. Within a matter of seconds, information stored in STM fades, unless we are actively thinking about that information or using rehearsal to maintain it in our conscious memory. Even then, anything that distracts our attention can force STM to discard that information in order to make room for something new.

2. Short-term memory (STM) is like a construction zone, because the information stored there is being actively processed and manipulated to solve problems and make decisions. That's why we also refer to this type of storage as working memory.

3. How much information can we hold in STM? At any given time, we can only focus on and process about seven items of information. Compared to the vast amount of information that bombards our senses every second, seven seems a very small number.

4. The limited capacity of short-term memory (STM) forms a bottleneck in the overall memory system, as shown in this diagram. Sensory memory holds an almost unlimited amount of information for a fraction of a second, but only a few bits of important information from sensory memory flow into STM. If that information makes it into long-term memory (LTM), there is plenty of storage capacity to hold that information for hours, days, or years.

5. Is there any way to stretch or expand the storage capacity of short-term memory (STM)? One useful technique is called chunking, which groups individual items into larger meaningful units. STM can hold about four chunks. Depending on the size of the chunks, this can dramatically increase the total number of individual items held in STM.

6. Consider these two examples of chunking. It’s almost impossible to recall a sequence of 11 or 12 unrelated digits, but the digits on the left can be chunked into three memorable groups of “traveling 11s.” The digits on the right become more easily remembered when chunked as a phone number—in this case, the telephone number for the Parliament of Canada.

How many items do you think an average person can hold in short-term memory (STM)? Two? Five? Ten? If you ask people to hold two or three items in memory for a few seconds, almost everyone can do it—people report that the task seems very easy. But, as you increase the number of items, the task gets progressively more difficult.

In a moment, we will test your memory for lists of 5, 6, 7, 8, and 9 numbers. On each trial, you will see a series of numbers appear at the center of your screen, one at a time.

After all the numbers have been presented, select the response box and type the numbers you saw in the exact order in which they appeared. Don't separate the numbers with spaces or any other character. When you have entered all the numbers you can recall, select the “Finished” button.

How much can people hold in short-term memory (STM)? The best estimate seems to be about 7 unrelated items, as George Miller noted in 1956 in a famous paper titled "The Magical Number Seven, Plus or Minus Two." Although the actual number of items that a typical adult can hold in STM ranges from 5 to 9, for most people and for most tasks, things become unpredictable after about 7 unrelated items. That's when items tend to get lost or "drop out."

When we say the capacity of short-term memory (STM) is 7 items, it's important to note that this does not mean STM can store only 7 numbers or 7 letters. STM can hold dozens of numbers or letters if they are combined through chunking into larger meaningful units such as telephone numbers, words, phrases, or sentences.