Chapter 88. Short-Term Memory Capacity

Learning Objectives

attention
the focus of conscious awareness; can be directed to particular events while ignoring others
rehearsal
actively maintaining information in short-term memory, by mentally repeating it or by making meaningful connections to other information
chunking
arranging information in units of manageable size to make it easier to remember; often occurs automatically
sensory memory
initial stage of memory storage; can hold a large amount of information from the senses for only a fraction of a second
consciousness
the experience of being aware of the external world
short-term memory (STM)
temporary memory storage; can hold about seven items for about twenty seconds
long-term memory (LTM)
relatively permanent memory storage; can hold a large amount of information for years
working memory
another name for short-term memory; emphasizes active processing of information
memory
the process of remembering information that has been learned
Short-Term Memory Capacity
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Learning Objectives:

Describe the amount of information that can be stored in short-term memory.

Contrast short-term memory capacity for unrelated items and for related items.

Understand why short-term memory forms a bottleneck in the overall memory system.

Review

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Review

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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.

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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.

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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.

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 The image demonstrates how our senses, such as sight, hearing, and touch, provide a lot of information that enters our sensory memory.  However, only a few pieces of that information make it into short-term memory.  Then, only a small amount of that information makes it into long-term memory.

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.

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The image includes two identical rows of shapes: 5 squares, 5 circles, and 5 triangles.  The first row has these shapes simply listed across the screen.  The second row has a circle drawn around each grouping of shapes to form three chunks or groups of items based on their shape.

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.

Review

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The image includes two sets of numbers.  The first set includes the following sequence of 12 numbers listed across the screen: 001101101100.  Below this sequence, these numbers have been grouped to form three chunks: 0011, 0110, and 1100. The second set includes the following sequence of 11 numbers listed across the screen: 16139924793.  Below this sequence, these numbers have been grouped to form four chunks: 1, 613, 992, and 4793.

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.

Practice 1: Demonstrating STM Capacity

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Practice 1: Demonstrating STM Capacity

Select the BEGIN TEST button to begin the memory test.

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.

Number of items in list Percentage correctly recalled
5
6
7
8
9

How well could you recall the numbers you saw?

Most people can handle 5 or 6 items, but make errors when the list has more than 7 items.

Practice 2: STM Capacity for Unrelated Items

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Practice 2: STM Capacity for Unrelated Items

Select the PLAY button to watch STM overflow when there are more than 7 items to remember.

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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."

Practice 3: STM Capacity for Chunked Items

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Practice 3: STM Capacity for Chunked Items

Select the PLAY button to watch the capacity of STM expanded through chunking.

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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.

Quiz 1

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Quiz 1

Drag the label for each type of memory storage to its correct location in this diagram. When all the labels have been placed, select the CHECK ANSWER button.

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Perhaps you should go back to review why short-term memory (STM) forms a bottleneck in the overall memory system.
Select the NEXT button and move to Quiz 2.
Long-term memory (LTM)
Sensory memory
Short-term memory (STM)

Quiz 2

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Quiz 2

Select the SUBMIT button after answering each question.

Anka was enjoying her vacation until she discovered her bank account was locked. A quick call to customer service assured her that she could reset the account with the 10-digit passcode shown here.

Passcode: 1149RST384

Unfortunately, Anka didn’t have any way of writing down the code. After she ended the call, she went to her bank’s website and tried to enter the code. After three incorrect attempts, she was locked out again, and had to make another call to customer service—AFTER she found a pencil!

1. What is the most likely explanation for Anka’s failure to remember the passcode?
She wasn’t familiar with bank passcodes.
The passcode exceeded seven characters.
She has a poor memory.
Some characters occurred twice.
2. What could Anka have done to increase her chances of remembering the passcode?
Repeat each character silently as soon as she heard it.
Focus on the last three items, which are most likely to be forgotten.
Try to visualize each character as she heard it.
Group the characters into three chunks (numbers, letters, numbers).

Conclusion

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